What exactly should I wear on a trip to Barcelona, Spain to blend in?

Style in Barcelona is quite informal, most girls wear jeans most of the time. and flat shoes / pumps like converse allstars. Simple tops. Not many dresses/short skirts unless its really hot. Nothing too ostentatious. Think san francisco rather than NY. Plain leather/fabric purse, or larger shoulder bag would be a good idea. But keep a good eye on it when on the beach, in cafes etc. money in pockets is a good idea too. I don't agree with the first answer about shoes from Spain, most "cool" young people wear the same old school trainers you'll see in the rest of the world Ah, and remember Barcelona is full of foreigners anyway.

Clothing is not that different from elsewhere in Europe. Smart casual is always good. Not too smart! Leather shoes are best. Purse. The good ones have strong, long shoulder straps and locals wear the strap over their heads with the purse under the arm.

Shoes. They must be from Spain. Otherwise, you will NOT blend in. And that means they have to be broken in before you leave, or you are going to be in PAIN! Of all the places I've been in Spain, Barcelona is the least formal. The kids wore torn jeans, cutoffs, tank tops, and their hair undone. Absolutely the LEAST Spanish place I've ever been inside Spain! (nothing wrong with it...great city, great people, incredible art, but the dress....totally unexpected!)