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