So say we've got an MC30-TF, with OE, running the 'opening salvo' objective as the second player.

He rolls his broadside attack - 2 blue and 3 black. Both blue come back 'crits', all blacks come back blank. Uses OE to re-roll all the blacks, they come back blank again. Decides to add the 2 dice from 'opening salvo' as black dice, rolls those...both blanks! Can the player use OE to re-roll those 2 dice, now? Or can OE be used to 're-roll all black dice' only once per attack?

No.

The Reasoning:

• A “while” effect can be resolved during the specified event and cannot occur again during that instance of the event.

Okay, makes sense. So switching the order around a bit...

He rolls his broadside attack - 2 blue and 3 black. Both blue come back 'crits', all 3 blacks come back blank. Decides to add the 2 dice from 'opening salvo' as black dice, rolls those...both blanks! Can now re-roll all 5 blank black dice at once, then, correct?

4 minutes ago, xanderf said:

Okay, makes sense. So switching the order around a bit...

He rolls his broadside attack - 2 blue and 3 black. Both blue come back 'crits', all 3 blacks come back blank. Decides to add the 2 dice from 'opening salvo' as black dice, rolls those...both blanks! Can now re-roll all 5 blank black dice at once, then, correct?

He sure can, yes.

10 hours ago, xanderf said:

Okay, makes sense. So switching the order around a bit...

He rolls his broadside attack - 2 blue and 3 black. Both blue come back 'crits', all 3 blacks come back blank. Decides to add the 2 dice from 'opening salvo' as black dice, rolls those...both blanks! Can now re-roll all 5 blank black dice at once, then, correct?

Sounds like one of my MC30 rolls...

Keep in mind, if you're going to spend a CF dial, to do it before you OE so that you can reroll the added die if you need to.

Similarly, if you're spending both a CF dial and token, spend the dial, then the token, then OE. You can't split the resolution of the CF before and after OE, because the CF dial+token is all one command.

Sounds like you need vader as well ( if your running imps).