Something that looks like a good thing at first, but has unforeseen bad consequences.
For instance, while irrigation schemes provide people with water for agriculture, they can increase waterborne diseases that have devastating health effects, such as schistosomiasis. (From wikipedia)
Major edit in body to provide a better example as requested in the comments. Saeid, please let us know if you disagree and this conflicts with your intent. -T.R.
In certain contexts, a poisoned chalice works.
An assignment, award, or honour which is likely to prove a disadvantage or source of problems to the recipient: "many thought the new minister had been handed a poisoned chalice" (http://www.oxforddictionaries.com/definition/english/poisoned-chalice)
When something is a curse in disguise (in the disguise of a blessing), you can also simply say that something is more of a curse than a blessing.
The example of the OP seems to be the "opposite" of a blessing in disguise in the sense that a setback had disastrous as opposed to beneficial consequences. We might call such a setback a disaster in the making:
in the process of happening
It became clear that this was a disaster in the making and we had no way of coping with it.
Cambridge Dictionary of American Idioms Copyright © Cambridge University Press 2003.
No one could have known it in advance, but in retrospect, that setback in car service was a disaster in the making for John Forbes Nash, Jr.
A 'curse in disguise' is the literal opposite, and isn't unheard-of. As in:
The phrase be careful what you wish for also comes to mind (Usingenglish.com)
If you get things that you desire, there may be unforeseen and unpleasant consequences. ('Be careful what you wish for, lest it come true.' and 'Be careful what you wish for; you may receive it.' are also used.)
Also see, this wikipedia entry on unintended consequences:
Unexpected drawback: A negative, unexpected detriment occurring in addition to the desired effect of the policy (e.g., while irrigation schemes provide people with water for agriculture, they can increase waterborne diseases that have devastating health effects, such as schistosomiasis).
... In CIA jargon, "blowback" describes the unintended, undesirable consequences of covert operations,
Blowback seems to fit
chiefly US The unintended adverse results of a political action or situation:
PS - I can't remember where, but I've seen the phrase Monkey's Paw used for this. A reference to the WW Jacobs short story, in which supposedly good things happen with disastrous causes behind them.
I would suggest that an unfortunate turn of events accurately conveys what you're trying express to here.
In an unfortunate turn of events, the car service that Dr John Nash had hired did not turn up. He hired a taxi cab which was involved in a crash, and both Dr Nash and his wife were killed.
Probably a Trojan horse may convey the idea of something apparently good or positive but that in reality is dangerous:
(Classical Mythology. a gigantic hollow wooden horse, left by the Greeks upon their pretended abandonment of the siege of Troy. The Trojans took it into Troy and Greek soldiers concealed in the horse opened the gates to the Greek army at night and conquered the city.)
- a person or thing intended to undermine or destroy from within.
( dictionary.reference.com)
The example given is not the opposite of a blessing in disguise, it's going from bad to worse. So you could say, in Nash's case, that "Murphy's law" applies:
Anything that can possibly go wrong, will
or you could say that he
jumped out of the frying pan into the fire
As Tushar Raj's answer notes, one traditional way to describe a bad result that at first looks like a blessing is in connection with the warning "Be careful what you wish for" (sometimes completed with the phrase "because it might come true").
A familiar instance of this warning is in the cautionary fable from Aesop of "The Frogs Who Wished for a King." The public domain Aesop for Children tells the fable this way:
The Frogs were tired of governing themselves. They had so much freedom that it had spoiled them, and they did nothing but sit around croaking in a bored manner and wishing for a government that could entertain them with the pomp and display of royalty, and rule them in a way to make them know they were being ruled. No milk and water government for them, they declared. So they sent a petition to Jupiter asking for a king.
Jupiter saw what simple and foolish creatures they were, but to keep them quiet and make them think they had a king he threw down a huge log, which fell into the water with a great splash. The Frogs hid themselves among the reeds and grasses, thinking the new king to be some fearful giant. But they soon discovered how tame and peaceable King Log was. In a short time the younger Frogs were using him for a diving platform, while the older Frogs made him a meeting place, where they complained loudly to Jupiter about the government.
To teach the Frogs a lesson the ruler of the gods now sent a Crane to be king of Frogland. The Crane proved to be a very different sort of king from old King Log. He gobbled up the poor Frogs right and left and they soon saw what fools they had been. In mournful croaks they begged Jupiter to take away the cruel tyrant before they should all be destroyed.
"How now!" cried Jupiter "Are you not yet content? You have what you asked for and so you have only yourselves to blame for your misfortunes."
[Moral:] Be sure you can better your condition before you seek to change.
The moral is the conclusion of the Aesop for Children compilers. An equally valid moral might be "Be careful what you wish for—it might come true."
Possibly "the devil's in the details" or "there's no such thing as a free lunch" — though you'd need a more realistic example to see if they'd be appropriate in your case.
Both convey the idea of a thing that's good at first sight but may come with a catch (though the second doesn't necessarily mean it was bad on the whole).
"Every silver lining has its cloud" is a turn of phrase I've heard recently too (I'll try to find a source) — though not a common expression, it's easily understood & could be used here too, I think.
You could says that it's a
Trojan Horse -
TFD definition
In my language there is even a better phrase, but I found out it doesn't directly translate into English / isn't well known.
It's something like "fear the Danaans, even if bearing gifts!"
More info here on wiki: Timeo Danaos et dona ferentes
The opposite of "blessing in disguise" which, as you have correctly pointed out, means a perceived misfortune unexpectedly turning into a fortunate event, would be a seemingly fortunate event turning out to be quite unfortunate. In this case we could say "even roses have thorns", or "not everything is as it seems", or maybe something else, depending on the situation. But the situation you have described is not a fortunate event turning out bad, because the source of the misfortune - the car not showing up - is not a fortunate event in itself.
Double edged sword is a phrase that could be used in this context to highlight that some things have two sides to them
