1998 chevy blazer After the repair shop replaced the blown intake gasket, I have no heat from the heater. I took the heater hoses off and ran water thru the heater but still no heat

At operating temp, check the heater hoses, with the heat on, if the hoses are both hot. exit hose could be a little cooler than the inlet side, still they should both be pretty warm.the coolant that goes through them is approx 190 degrees to the inlet. if they are not hot, the coolant is not circulating through the core or the thermostat is stuck open, or its low on coolant

You took the hoses off the heater and ran hot water through the heater and it still wouldn't heat??? Is the fan running?
Ignoring your diagnostic attempts, I would say they left out the thermostat, or possible crimped a heater hose, or pulled off the water valve actuator cable or line. If you ran hot water through the heater core and still didn't get any heat, the laws of thermodynamics have abandoned you.

Is the vehicle running at normal temperature..If its running cooler than normal,then it might be the thermostat..Feel the upper radiator hose after the vehicle hot..It should be hot to touch