Is Serbian u from "buva" flea a Proto-Slavic ъ or l from *blъxa?

Likely both. In Slavic languages, sounds corresponding to Proto-Slavic lъ/ъl often go together as a unit and can change places in different forms of the same word. So do lь/ьl, rъ/ъr, rь/ьr.

Serbo-Croatian's "dark" pronunciation of l makes the lъ/ъl combination sound very much like "u". There's a pattern in the spelling of words like vuna (wool), puno (a lot), Bugarska (Bulgaria), gutati (swallow), etc where the corresponding Proto-Slavic forms had lъ/ъl/lь/ьl.