You can reference all of Drokatta's abilities that I mention here: http://cards.boardwars.eu/Expansion-Boxes/Heart-of-the-Empire/Rebel Heroes/Drokkatta/

So let's discuss how this card actually works, in particular whether it can still applied even if the attack does 0 damage to the target and if it can combine and reduce multiple instances of Blast 1.

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Exhaust this card while attacking to reduce the Blast by X to a minimum of 0. A figure other than the target within 2 spaces of the target suffers X[damage].

Emphasis mine. The first bolded text is what makes this ability problematic because of how Blast is applied in the rules, which is done completely after the attack has resolved. But "while attacking" implies that this ability is used before the attack has resolved, which means it does not require damage to be dealt and takes effect immediately upon use. But then we run into this little gem.

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RRG, p24: A [surge] ability that relates to a keyword, such as Blast, Cleave, or a condition, applies after the attack resolves.

Oy vey. So if Blast is only being applied after the attack resolves, how can Shrapnel Rounds be used "while attacking"? And does this mean that Shrapnel Rounds requires the target to suffer damage in order to apply Blast and then reduce that Blast to use Shrapnel's effect? Let's look at the rules for Blast.

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RRG, p7: If the target of this figure’s attack suffers one or more (damage), each figure and object adjacent to the target space suffers damage equal to the Blast value .

This rule mentions a cryptic "Blast value" that is referenced when applying Blast damage, so perhaps this is the value that the first sentence of Shrapnel Rounds is referring to and exists during the attack. So that would lead me to believe that Shrapnel rounds can indeed be used before damage is dealt and does not require the target to suffer damage the way that Blast does.

However that leads into the next question, which is what happens when an attack has Blast coming from 2 separate sources? For example if Drokkatta is triggering the Blast 1 surge ability from her Boomer weapon as well as the inherent Blast 1 from Shrapnel Rounds? There is nothing in the rules that states that Blast effects are combined. Obviously they stack, but I swear I have heard @Pasi say that separate Blasts are applied separately, which allows for strange corner cases like an Elite Royal Guard avoiding the 2nd Blast by moving away from the target space with Forward Vengeance. So can Shrapnel Rounds combine multiple sources of Blast and reduce them for a cumulative damage effect?

I usually try to do my own research when figuring out a rules interaction, please let me know if I'm missing anything. I'm thinking this might just not be covered in the rules and need clarification from FFG. Both questions have a huge impact on how good the card is. Hoping to get this cleared up before we start our Heart of the Empire campaign in January.

With the current best knowledge:

1. As written you can use Shrapnel Rounds even if the blasts would not have been resolved had they not have been removed by Shrapnel Rounds. (The only slightly problematic thing is that this applies even when there is a miss due to insufficient accuracy. But to change that would need an errata.)

2. Each blast is resolved separately and they are not combined. However, reducing Blast by X can easily be interpreted as "removing blast effects worth of X", which is obviously what the ability has been intended to do. Entry in the next FAQ is expected.

Edit: Note that Blast is activated/added during the attack but resolved after the attack resolves. If you add or remove Blast effect(s), you must do that during the attack.