Is possible to use this card Blinding Speed on a knocked out hero? In that case what happens, a regular attribute test?

A knocked out hero is not a hero. He is only considered a hero if you're targeting him with an ability that recovers damage. Therefore, you cannot direct this card (or any ability) at a hero token unless that ability is a healing ability. (See the little box in the top right of pg15, "knocked out heroes.")

Ty for the lightning response!

Ty for the lightning response!

you could say Zaltyre replies with...

blinding speed!