Sensor for Black Color...

Can anybody help me how to build a sensor that can distinguish between black and white...my aim is only to build a low cost sensor that can detect black color only? then disregard the white ones. Is this possible to build? any idea is such a big help for me...thx...

An LED and a photo-diode/transistor is all you need, you can even buy them in a single encapsulation for this exact purpose - try googling for 'line follower'.