By - Klondiker1
When your opponent declares a shooting attack, but before the target is selected, you use Blinding Light. If that renders that operative unable to be targeted, then your opponent chooses another target to shoot. If there are no targets that are valid to shoot, the shoot action is cancelled and the action points are refunded.
The shoot action is not cancelled, it is still performed, it just fails to resolve.
That wording is important, as it denies the ability to do something else with that first AP and then attempt the Shoot action again.
There are quirks to this, first, because it specifies that it must be a different action, this slaps multishooters like Marines, as they must now perform something that is not shooting with the first action, and can't elect to use their second shooting action instead of the first shoot action (so they will only be able to shoot once this turn).
Second, if a multishooter like Warpcoven were to use their pistol as their first shot and you Aura it, they can immediately choose to cast Doombolt and shoot with that (then do something else with the second AP because they've already shot twice). Note that they still can't shoot the Aura girl, as she remains in cover until the end of the activation.
Third, caster-shots like Doombolt get wrecked if you tag them, because they spent the AP on the cast action which triggered a free shoot action, so they don't get a refund, they've still used the cast, and they still can't follow it up with the shoot action.
This is how I was originally playing it based on previous rule iterations within 40k over the years. In the interest of maintaining a fun environment I'll avoid this complexity until it's Faq'd as Im not 100% certain this is what they intended with it.
I suspect RAI was the model is not valid and the shooter can pick a new valid target or loses its apl spent if no other target is valid.
That is RAW as well: you're forced to choose another target if one exists that they could shoot at, and if they can't the action ends and they're refunded the AP used to perform it.
My explanation is assuming no other valid target is in LOS, which is often the case.
The rules lawyer use case would be using it to delete grenades, as specific (act of faith) overrules general (weapon keyword), allowing you to negate Indirect, and then they also lose the grenade because Limited triggered when they selected to use it.