The juvenile is similar to the female, but has pale spots on the upperparts, and the very young juvenile also has a speckled breast. Young birds vary in the shade of brown, with darker birds presumably males. The first year male resembles the adult male, but has a dark bill and weaker eye ring, and its folded wing is brown, rather than black like the body plumage (description taken from wikipedia)
I've been seeing this young blackbird regularly hanging around my front yard over the last few weeks. It's starting to get the black feathers of the adult now.