The original form is baštu rather than baltu (George 2016, 113f.). No etymology can be suggested.
balu, balāt - without, without the consent of, apart from, in the absence of
(CAD B 70, AHw. 100)
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).
bālu - “nothingness” (designation of the planet Mars)
(CAD B 74)
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).
balû - to become extinguished, to come to an end
(CAD B 72, AHw. 121)
Var. belû. Derived words: belû 'extinguished' (CAD B 94, AHw. 121).