This paper suggests that TEA only blocks N-type inactivation when applied intracellularly.

http://www.pnas.org/content/88/12/5092

TEA blocks many K+ channels so be cautious in using it

Perhaps my phrasing was unclear - I meant that TEA only blocks N-type inactivation when it is applied intracellularly as opposed to extracellularly, not that TEA blocks N-type inactivation and nothing else when applied intracellularly. You are correct that it blocks many K+ channels, I was assuming this would be in culture if the goal is to test a specific type of inactivation.