Pfitz, black market items exist here in Spain and in fact is a nuisance that the police are trying to crack down on. Most of the black market vendors are illegal immigrants who sell their wares spread out on blankets that they can quickly gather up when the police come through on their regular sweep.
The top product being sold this way are bootlegged copies of CDs and DVDs. However, as you can imagine, there is a little bit of everything...perhaps not as exaggerated as Canal Street in NYC. In Madrid you will find these street vendors in the Puerta del Sol, Calle Preciados, as well as highly trafficked metro stations and anywhere touristy.
In Barcelona you will find them around the tourist destinations as well as the Ramblas. However, when I was in Barcelona this summer, I noticed the police really being strict about those who do business on Las Ramblas ... asking to see business licenses etc.
Be careful when you shop in these areas as they are also rampant with pickpocketers who are very very good at their craft.