Had to use a block sealer for a cracked head gasket over the summer, it has been 5 months. And it is still running fine. Now that it is cold outside I tried to run my heater but it is not blowing hot air. Would the block sealer possibly clog my heater core and if so is it OK to flush the heater core?

I have no experience with those sort of products; however, what you say sounds entirely possible. I would flush the heater core--you have nothing to lose.

First, please tell us about your car.

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

Flushing may not be effective if the core is already blocked.
Depending on your vehicle's system, you might check to see if the system is capable of switching to the Heat mode.