What happens if I put a Keldeo GX with "Pure Heart" Ability into play after my opponent has used Latios GX's "Clear Vision GX" attack? Would my Keldeo GX be able to use its "Resolute Blade GX" attack or not?
As long as the Pure Heart ability is not blocked or turned off when Keldeo GX comes into play, it would be unaffected by Clear Vision GX.
Source: TPCi Rules Team (2019-12-05)

Relevant cards

Note: All applicable cards may not be listed below.
SM11_219
SM11_219
SM11_223
SM11_223
SM11_240
SM11_240
SM11_243
SM11_243
SM11_47
SM11_47
SM11_78
SM11_78