"Blood red" can be both a noun and an adjective:
Blood red is my favourite colour. [noun]
The wall was blood red. [adjective]
The "blood" is optional in the sense it can be removed without grammatical effect:
Red is my favourite colour. [noun]
The wall was red. [adjective]
When "blood red" serves as a noun, then "blood" is a noun that's modifying another noun, "red", attributively (i.e. in the same phrase) and is optional, so by my understanding "blood" is acting as an attributive noun. But when "blood red" is an adjective, "blood" seems to be a noun modifying an adjective instead. Is "blood" still an attributive noun in this situation?
My question also seems to cover compounds like "brick red" or "forest green" (though I suppose not "royal blue" or "deep velvet" and presumably not "racing green" — is this correct?) and is somewhat related to this question about hyphenation.