I cannot answer either of your questions. You can check blacklisting at dnsstuff.com or similar sites. As far as unblacklisting, I expect if you have a Comcast business class account you won't encounter this problem and could rightfully request your IP to be "clean". If you have a residential account, then by definition it has a dynamic IP address and may be blacklisted.
I do not know if Comcast supports the blacklisting or if it just evolved by the sheer quantity of Comcast IP's out there and and number of unauthorized servers or spammers using their IP's.