1995 Chevy Blazer Temp gauge reads 240

I recently changed the heater core in the blazer. Also replaced the thermostat and heater hoses. Before the change the gauge would read 200-210 and stay steady. Now the gauge will go to 240 and does not blow hot air at all. If I feel the heater hoses, they are cool, the upper hose sounds like air is in the system, the lower hose is warm but not unbearable. I am wondering if I might have a blown head gasket on the exhaust side. I tried the block tester but had to put it in the overflow tank not the radiator as I do not have the adapter. When testing this is remained blue but still think it needs to go in the radiator not coolant tank.