Unfortunately, Comcast has installed this malware-like feature which blocks good sites and allows no way to override the block. Go to your xFi (internet) service, and click on More and then My Services. When xFi Advanced Security comes up, disable it.
Why Comcast is using such a flawed service is anybody's guess. And the block screen doesn't indicate where the service is originating from so that the poor user knows where to look for a fix. It's not a malware download. It's Comcast.