Neither term is correct for what. What (like how) is a Wh-word that can't be used as a relative clause marker. For example, the following are all relative clauses marked with Wh-words; the last two are ungrammatical -- in practice, we use that instead of how to mark relatives.
- the man who did it
- the place where they did it
- the day when they did it
- the reason why they did it
- the thing which they did
- the way that they did it
- **the thing what they did*
- **the way how they did it*
As for the difference between "relative adjective" and "relative pronoun", pay no attention; relative markers can function as both. In fact, pay no attention to what "Part of Speech" a word is said to be; the citations are wrong, they're not using the correct list anyway, and, in the first place, most English words belong to several different categories.
However, since this is not a relative clause, this must be a different what. It is in fact the Wh-word that introduces an embedded question subject complement.
- what little blood was still visible
which is a noun clause that is the subject of appeared in the quotation.
Embedded question complements are finite (they require a tensed verb like was), and they must begin with a Wh-word (there is in fact a special Wh-word whether, which is used only in embedded questions to mark an embedded Yes/No question.
- Will she be here?
- I don't know whether she will be here.
- Whether she will be here is something no one knows.
This kind of embedded question refers to the answer to the question, whether it's known or not:
- What they did was terrible. = The answer to "what did they do?" is terrible.
- What they did is unknown. = The answer to "what did they do?" is unknown.
And, to get finally to the point, in this case it's a special use of what, quantifying the amount of blood and indicating its degree.
- what little blood was visible = The answer to "what amount of blood was visible" is "little".
Other synonymous constructions that can be substituted in the same context include
- the small amount of blood that was visible
- the small amount of visible blood
- what visible blood there was
- what blood there was (that was) visible
I have since come to believe that there is no semantactic difference between "headless relative clauses" and "embedded question clauses". Whatever distinction there may be becomes ever less pronounced the more the clauses are dismembered, especially when they reach skeletal infinitives like What to do or How to do it. I just call them all "Wh-clauses" any more.