It seems to me that if one describes hell as 'bloody', that is simply describing one of the properties you'd expect of it. So, why is 'bloody hell' used as an offensive or shocking phrase?
There are two ways of looking at it. One is that "bloody" is (or was) considered vulgar and appending "hell" - which is also a taboo subject - is a kind of intensifier.
Another way to look at it is that religious references used as an exclamation have traditionally been considered profanities. The word profane itself means taking something sacred and using it for an unsacred purpose.
Whether Rawson finds it far fetched or not, "bloody" for "God's blood" would be a profane thing to say, because "blood is the life" according to Old Testament - so making an oath on God's life would be very serious indeed. As a matter of fact it is arguably a violation of one of the ten commandments.
The combination of "hell" (which is the antithesis of God) with God's blood therefore has the makings of a pretty blasphemous expression. There was a time when such a thing would be considered offensive.
Nowadays it's only "mildy vulgar" according to Wiktionary. But why was it ever vulgar?
Etymonline.com says of bloody:
It has been a British intens. swear word since at least 1676. Weekley relates it to the purely intensive use of the cognate Du. bloed, Ger. blut. But perhaps it ultimately is connected with bloods in the slang sense of "rowdy young aristocrats" (see blood) via expressions such as bloody drunk "as drunk as a blood." Partridge reports that it was "respectable" before c.1750, and it was used by Fielding and Swift, but heavily tabooed c.1750-c.1920, perhaps from imagined association with menstruation; Johnson calls it "very vulgar," and OED first edition writes of it, "now constantly in the mouths of the lowest classes, but by respectable people considered 'a horrid word', on par with obscene or profane language."
The onset of the taboo against bloody coincides with the increase in linguistic prudery that presaged the Victorian Era but it is hard to say what the precise cause was in the case of this specific word. Attempts have been made to explain the term’s extraordinary shock power by invoking etymology. Theories that derive it from such oaths as “By our Lady” or “God’s blood” seem farfetched, however. More likely, the taboo stemmed from the fear that many people have of blood and, in the minds of some, from an association with menstrual bleeding. Whatever, the term was debarred from polite society during the whole of the nineteenth century. [Rawson]
Shaw shocked theatergoers when he put it in the mouth of Eliza Doolittle in "Pygmalion" (1914), and for a time the word was known euphemistically as "the Shavian adjective." It was avoided in print as late as 1936.
Etymonline says of hell:
As an expression of disgust, etc., first recorded 1670s.
Add a swear word to intensify a taboo expression of disgust and we get bloody hell.
Well, in Australia 'bloody' is merely an intensifier. You hear expressions like 'bloody hot', 'bloody heavy', etc. It can even be inserted into words, like the infamous name of a hotel, the 'Inter-bloody-continental' ( also 'Interconti-bloody-nental'. The term 'bloody hell' is usually used to express astonishment and disbelief: 'He wrecked his brand new car' - the other person says 'bloody hell, he was probably pissed as bloody usual'.
Whether the idea that 'bloody' derives from 'by our lady' has any substance or not, people in Britain certainly use the word as though it did have. As well as an intensifying adjective it is also used as an exclamation. This is from where the 'Bloody Hell' of the OP originates.
The expression, so the legend goes, was once 'By our Lady Mary', shortened to 'Bloody Mary' and given as an exclamation following some shocking news. e.g. 'Ten of ours are dead', 'Bloody Mary'. People, it is said, came to substitute other words for Mary, e.g 'hell', 'wars' etc.
Cockneys today will sometimes use it with an alternative girl's name. The name chosen is invariable one that has had its day. Hence you will frequently hear 'Bloody Hilda', or Bloody Phyllis' used as exclamations. 'Crystal Palace beat Chelsea 5-0', 'Bloody Margaret'!