1) Vegeta's hair isn't black, it's dark brown.
2) Hybrids can obviously go super saiyajin, it happens all the time. If Trunks can have PURPLE hair of all colors, then the possibility of a blonde isn't far fetched.
Yes, I think it would just become spikey, maybe become darker blonde if it was light already (although it sometimes ends up going quite white when they're fully powered up). A super saiyajin's aura/glow makes their clothes seem to be a different color too (more filtered/shaded), even when they're not powering up, so you'd be able to tell from that and the eyes as well.
Toriyama himself made fun of the idea in Neko Majin Z by having a Saiyajin with very short hair (almost shaved off completely) going super. If I remember correctly Neko Majin said there was no noticeable difference and the guy pointed at his head and said "no, look, it got slightly spikier" or something.