Captain America had already been filming for a week when the Comic Con started. However, I believe I heard somewhere Chris (at that time) had not yet begun actual filming himself. Maybe I am wrong about this, or perhaps I misunderstood something. I know for sure he did costume testing on film by that time, and I believe it is the footage they showed of him at the Comic Con. However, I don't think at any time any of the footage showed him as Steve. Besides that, the only other footage I believe they showed for Captain America was of Hugo as Schmidt entering Odin's tomb and finding the Cosmic Cube. So, if I am correct, then it is still entirely possible that Chris may have Blonde hair as Steve Rogers. Either way though, this movies success will not depend on the color of his hair, but more importantly the story and portrayal of the characters.
Surfer