What Inner potential should I try to get?

I got +8% of damaga against tagerts with (in my case) blindness, thats when I realized it might be equal to +8% total damage or something considering any boss I'll be fighting will have blind from panic on it.

Is it right to assume that 8% damage on blinded enemies = 8% total damage?

In the same way as ACA, Enrage and Chance Attack, it is a flat multiplier to your damage, whereas the Total Damage% you get on weapon/secondary potential lines and Kanna/DA link skills become less effective the more you have.