Not sure about BCN (but the obvious place to see is Picasso museum), but in Madrid, contemporary art is usually showcased at Museo Reina Sofia
The La Caixa Building also spots a lot of exhibitions
If art involves photography, there are quite a number of galleries scattered around, this is one that a friend of mine works for:
Blanca Berlin Galeria
Another friend has a gallery showcase of his own photography somewhere near Retiro Park, but I?m not sure if he?s still there
Ricardo Cases
As for underground, I?m not in the art world, but there is quite a lot happening around the neighbourhoods in Madrid. Probably best place to start is in Chueca!
Hope this gives you a little bit of an idea… sorry that I can?t be of much help!!
Edit: sorry moderators, didn?t realise links aren?t allowed.. I?ve removed them.