It’s a bit surprising that this hasn’t been found or identified before, but apparently if you dial an emergency number, you have to wait 48 hours before being able to block any phone number. In addition to not being able to block any phone numbers, it appears that you will have no data connection for a period of time after dialing the emergency number.
It’s not known when this feature was added, but the Dialer application from Google was recently updated, so it may have come through with the update. Maybe it was just another one of those back-end switches that Google finally decided to turn on.
This move was more than likely put in place so that you (or someone else) can’t block emergency numbers, giving the authorities the ability to call you back if something goes awry. Regardless, when put in the right context, this is a great move by Google and hopefully this helps someone out if they are in a sticky and unfortunate situation.