If my opponent has used Latios GX's "Clear Vision GX" attack, would my Keldeo GX be able to use its "Resolute Blade GX" attack?
Yes. The effect of Clear Vision GX is placed on each of the opponent's Pokemon, and since Keldeo GX's Pure Heart Ability prevents all effects of attacks done to it by your opponent's Pokemon GX, it is unaffected by Clear Vision GX.
Source: TPCi Rules Team (2019-09-05)