As a synonym of bēlu “lord”, the present form must go back to WS *baˁl -. Following SAD 1 7, one has to assume that a nisbah suffix was attached to *baˁl -, even if this is rather unusual.
It is conceivable that Akk. ballukku and Sum. (šem)bulug were borrowed independently from a third language.
Probably of Sumerian origin (perhaps a “learned” loanword), but the etymon is obscure.
The original form is baštu rather than baltu (George 2016, 113f.). No etymology can be suggested.
See related forms with the suffixed -t: OA, NA balāt 'apart from' (CAD B 45, AHw. 101), OA, NA balāt 'apart from' (CAD B 45, AHw. 101), MA balut 'without' (CAD B 76, AHw. 101).
No definitive etymology can be suggested. In Oelsner 2005–2006:121 and Horowitz 2014:234, fn. 1584 it is proposed to derive bālu from baˀālu 'to be large'. This interpretation, although semantically plausible, is hampered by the absence of the “strong aleph” in bālu (one would expect *baˀlu).
Var. belû. Derived words: belû 'extinguished' (CAD B 94, AHw. 121).