How old is your bike and how much have you ridden it outside? Right now up here in the PNW even a little outdoor riding can gunk up your bike pretty darn quick, since that ice storm we had meant a lot of roads were sanded and none of it was cleaned up (at least around here it wasn't).
Generally if I've ridden and its been at all wet (this creates that lovely fine grit slurry that you are seeing on the road and throws it right up into all your bike parts) I rinse down the bike and dry it thouroghly as soon as I come home. Even if it hasn't been wet you may want to clean your chain fairly often with all the sand that is out there right now.
A really fast build up of thick black gunk can indicate that you are over oiling your chain too. Ideally you want to wipe most of your excess lube off after it has had a chance to penetrate the chain.