"Block heater" seems to be a bit of a misnomer. Generally they don't really heat the block. Instead the aim is that the oil is warm enough to flow freely around the engine to reduce wear during cold start-ups. The engine still takes a while to heat up when you drive away - it isn't warm enough to run the heater immediately, for example.
Where I am (Finland) many cars also have a small mains fan heater in the cabin to heat it up. These work well. Our cabin heater is, I think, about 500. I don't know the power rating of our block heater, but the two heaters (cabin and block) run off the same electrical supply which is a standard domestic socket. I think all the sockets used for car heating are the same rating as a standard domestic socket. Not high-rated supplies suitable for fast EV charging.
A related issue is that in winter most people(*) will still want to run a cabin heater, so that would reduce the current available to charge an EV.
A minor problem is that the supplies are generally on timers, because you only want to heat the car/engine just before you go out. You don't want to heat the car all night. On mine, I hit a button to run the heaters for a half hour, 1 hour or 2 hours before I go out. Others are on those old-style rotary timers like you used to get on emersion heaters. The issue would be that you would still only want the heater running just before you go out but the charging would need to run for a longer period. Not an impossible problem to solve, but the existing supplies could need updating.
(*) I say most people because not everyone uses mains heaters, and not everyone has the option. Also not every car has these heaters fitted. One of our two cars does, and one doesn't.