When someone says "Nothing too [X]," it usually means "It will not be good if it is very [X]."
In the context of your example, "Nothing too elaborate" means "you do not want an elaborate blouse." (You want a plain blouse, not a fancy one.)
Here are a couple more examples:
"I'd like to order dessert, but nothing too sweet." (Meaning: I want to order dessert, but I want a dessert that is only a little bit sweet, not too sweet.)
"I'm just looking for a basic computer. Nothing too expensive." (Meaning: I don't want a computer that is very expensive.)