The heater valve could be failing to open for one reason or another.
It could be a blocked heater core if your fan works... Or it could be your thermostat... I have an 03 that is doing the same thing. I just replaced the thermostat and it is still doing it. There is a site called BlazerForum.com you may get some better answers there
Check coolant level .Then thermostat . Then hearter core
this model does not have a heater control valve, it has a specail thermostat that controls flow.
how to replace a heater core in a 1997 Chevy blazer
About $9.00
The thermostat might need to be replaced. or the coolant level mite be low. it is a sign that it is. and also the type of coolant used if not flushed and replaced tends to clog the heater core. try flushing it out. lastly you have a bad intake gasket and leaking coolant out. 4.3's are notorious for this.
probably the blower motor resistor
No.
Factory said to use a 195 degree thermostat
Either a faulty thermostat or you have a low coolant level.
Your 1999 Chevy blazer doesn't have a timing belt.
Same as a 96 blazer go here.How_do_you_replace_the_blower_motor_on_a_1996_Chevy_blazer