That Cure-ish sound, which I'm hearing in Cocteau Twins' "Iceblink Luck", is that a guitar or a bass or what?

6-string bass, yo.

Definitely with chorus and reverb.

Yeah, Cure did six-string bass with chorus. Simon Raymonde plays regular four-string bass with chorus (and often some flange), but he spends a lot of time up in an equivalent high range. Other thing that bulks up that chorus effect: playing "chords" on bass (or in these bands cases usually one string ringing open while fretting high on the next one).

Thanks.