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:
Etymonline says of hell:
Add a swear word to intensify a taboo expression of disgust and we get bloody hell.
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'!