The infinitive bluć, although now obsolete, is relatively new.
The original form is blwać : bluję, blujesz, ..., blują, bluj, ...
The corresponding Old Czech verb is bľvati : bľuju, bľuješ, ..., bľujú, bľuj, ..., after umlaut u > i (and palatal ľ > middle l) bliji, bliješ, ..., blijí, blij, ...; nowadays with a new infinitive blíti, formed analogically to bíti (biji, biješ, ... = to beat).
Related to Lith. bliūti, bliauti = блеять, мычать, ...;
A similar Pansalvic verb (b replaced with p):
Rus. пльвати > плевать, Pol. plwać > pluć, Cz. pľvati > plivati (plivu) and plíti (pľuju > pliji), ... = to spit;