Blast is a special ability for military units making gains increased damage output for each tile closer to the target and ignores Stealth.
However, one must note that the AI, when using artillery units with the Blast ability, will move the units closer to the target. This is very important to note when using Blast Units, as this can lead to worse results than anticipated especially in auto-battles. Similarly, you can also spot this in manual fighting against a defending army with Blast Units.