Most of them are blacklisted by Sorbs. If you enable Sorbs RBL, it will bounce. This is fairly recent. My guess is Comcast wants you to use their mailservers so if you run your own or try to relay through a server with a Comcast IP, you have to take steps to avoid the bounce.
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.
one of my clients at work is using microsoft's frontbridge as a archiving solution -- of which their outbound emails go through frontbridge. about a week ago, they started getting comcast bounces and when we contacted FB, they said they were blacklisted on comcast's list. they (FB) said it has since been removed but the customer forwarded me another bounce back over the weekend ...
I doubt if the Comcast csr would know what blacklistings are. I recently switched to Comcast telephone and their callerid database is woefully out of date. I called customer service and he said it was the problem of the other providers (AT&T and Charter). When I had AT&T all the numbers were correctly identified!
Re the Comcast email..I expect if you use smarthost (sendmail) or something similiar and relay your outgoing mail trough Comcast mail servers you won't have any problem.