Did some research on this based on information in the comments.
There appear to be two schools of thought on the etymology of the adjective "sucks".
The first is that it comes from relative innocuous roots (one possible example: the phrase "suck eggs", which can be found in the works of Mark Twain).
The second is that the roots of the term are in its older meaning of fellatio. For example, Etymology Online's entry says:
Meaning "do fellatio" is first recorded 1928. Slang sense of "be
contemptible" first attested 1971 (the underlying notion is of
This better matches the traditional intent of the word (as a longtime user from at least the mid-70's), as well as the variants often heard in the wild (which invariably are elaborations on the theme of oral sex). Still, it would be tough to say which, if indeed either, interpretation is correct.
As for "blows", I couldn't find much. Supposedly it originates from the USA in the 80's. I did happen to be a big user of the word myself, as a teenager in the early 80's. I know my peers (in the middle of the USA) used it as a nearly exact synonym for "sucks", for those occasions when that word had been used enough that its impact may not quite be as hefty as desired. Really its best to alternate.
As a near synonym, it had the same vulgar connotations to us as "sucks". However, it is true that one could tack on "monkey chunks" to transform it into a slightly different vulgarity (making it a bit more versatile, and thus preferred by some of my fellow connoisseurs of angst). I can't for sure say if the "monkey chunks" predated the lone sense of "blows", so exactly which of the two bodily actions it was originally intended to imply can perhaps only be discerned through further research. Possibly a good dissertation for someone in the offing here.