First of all we need two vectors: v1 = normalize(grenade position - character position) and v2 = character camera forward vector.
Let a = dot product of v1 and v2 vectors. Then -1 <= a <= 1 (because v1 and v2 are normalized). We can add 1 to this value and then multiply it by 0.5 to get some value between 0 and 1. And the more this value will be the more angle between character camera forward vector and (grenade - character) vector was (and the less blind effect should be).
Now we can interpolate this value using Lerp node where a = 0 and b = this value with FlashbangTimeline output as alpha (which is based on FlashbangCurve value) to get this fade out effect.
This is the basic of blind power calculation. Beside that, there is some extra math to determine obstacles between character and grenade.
Let me know if you need more detailed explanation!