I accidentally blocked someone from my ship's comm. How do I unblock them?

There is no way to do this yet in-game. You can file a support request to unblock someone, however.

How about the block symbol changes color when they are blocked and to unblock them you just click it again and it changes back?

If you delete the message though (and we're assuming they are blocked due to something offensive etc), that block/unblock link is gone though. Although, Dan could implement somewhere a "blocked list" next to "Comm Archive" in the Legion tab. There's plenty of room for it.

That would be he simplest solution to the problem. There definitely needs to be a blocked list so you can at least see who you have blocked over time. It may be that weeks or months later you will need to communicate with the person only to find you can't. Without a block list you may not remember it's because you blocked them previously.