I would suggest asking comcast what it means. After all they issue the email address and it is their servers the email is sent from. Thunderbird is only a client sending to their server when you click send.

I can go directly to Comcast and get my mail and the message I get is from Thunderbird where I use the same password.

Thunderbird is just a program on your computer, it has no knowledge of what happens outside of the program.

If you keep getting messages saying your email addresses have been blacklisted and want me to change passwords. These messages are not from Thunderbird. They are being received into your mail account.

Are you getting any other mail into those accounts ? Are you able to send messages from either of those accounts.

Changing passwords does not stop you from being blacklisted. So it is possible the emails you are getting are from a scammer who is trying to get you to tell them your password.

See if your email address really is blacklisted. https://check.spamhaus.org/