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).
The only transparent cognate is Hbr. bāmā “open (hilly) country, hill, highland”.
Var. bantu, pandu.
Cf. bamāniš 'in halves' (CAD B 76, AHw. 101).
No definitive etymology can be suggested as no clear Semitic cognates with comparable meanings have been detected.