Ooooooooohhhh...kay.

Boba rolled 2 hits, 1 crit and 1 blank at range 1. 3 ties infront of him. Can he re-roll that blank die 3 times till he gets a good output? Does the same thing goes for defending aswell?

Thanks.

No.
You can only re-roll a die once per attack.

His ability lets you re-roll up to 3 dice in that scenario, though.

As mentioned you only get to reroll a given die one time.

If Boba also had Predator and chose to spend a TL on his target he would still be limited to rerolling a given die a single time despite the appearance that he could reroll multiple times using Predator, then TL, followed up by his pilot ability three times following the original post. Boba Scum's ability is little different from Han's ability going back to wave 2.

Incidentally, the rule fire this is on page 12 of the rulebook, in the black modifying dice box, last paragraph.