Comcast tor saga continues. Found out that the blocking is probably implemented by router/modem provided by Comcast. And can be turned off if one is determined enough, and doesn't mind scary warning pop-ups and weekly mail reminders. It is included in their "Advanced Security" feature. Because if random IP address connects to your computer, it is ok, but if a tor relay does, it is automatically a security threat, so should not connect to your computer. At least Comcast geniuses think so. Less evil because of possible opt-out, but still evil, because it has little to do with security and discourages people from running tor relays.

(Sources: 2 Comcast customers contacted me and told me, and we did some tests.)