One who says the blessing “Baruch she’amar” and finishes it together with the cantor may not respond to the blessing with the word Amen, as it looks like he is saying Amen to his own blessing. (Shulchan Aruch and Mishne Berurah 51)
Since the blessing “Baruch she’amar” is the introduction to the verses of praise, one may not interrupt between the blessing and the verses. For these purposes, even one word is considered an interruption (and it could be that one must start the blessing again), and even if one remains silent it is correct not to wait too long. Instead, immediately after “Baruch she’amar” one should commence the recital of the verses of praise. (ibid)
