1999 blazer 4x4 lacking heat during cold days

Do you have a check engine light on ? If so Get codes first for a direction. Sounds like temp sender or coolant component is faulty.

It sounds like you have too much water mixed with the antifreeze, or your thermostat needs to be replaced. If this doesn't help, have the water pump replaced.

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When your engine thermostat has failed or is sticking open, the engine fluid isn't able to reach a temperature that lets your heater core get hot enough to produce worthwhile heat to blow into the passenger's compartment. It is also not good for your engine to run below design temperature.
If you have a dashboard temp gauge, if your thermostat is stuck open, that gauge will probably stay in the cold area.
I would think that that is the problem, and I would have the thermostat changed (not a very difficult or expensive job).
Hope this helps you.

flush the radiator and cooling system and refill with fresh coolant and distilled water. i know during extreme cold temps, it will take a while but try some good maintenance and see if the situation improves. happy new year

no check the thermast

This sounds like your automatic climate control is starting to malfunction. Check to make sure your coolant is full, and temp is not fluctuating, and when your vehicle is warmed up, try using the low setting and feel for air temp. Is it warmer than if you turn it up? When at idle the vehicle is not producing heat the same as if you are driving, putting load on your engine and more air/fuel combustion means more heat, which is why you can feel heat while driving. Ideally you should feel heat as soon as the engine is warming up, getting hotter as it reaches peak temp of 190-210 degrees. You can try to make sure your water control valve is not blocked, and all electrical connections are secure. This will be located close to your firewall and connected to the coolant tubes that run into your firewall to your heater-core. Also use recirc (setting)to try to summon as much heat as possible, however ultimately, you may need to take you vehicle in for service. If it only warms when driving defrost will be an issue. Good luck.

if you turn the temp switch to cold and then to hot and it stays the
same you could have a blen door stuck.If you have a little heat go to a radiator shop and get a power flush they are the only one that can do a good flush

cracked cylinder head or blown gasket

If the engine is not over heating then it is doing its job. The constant cycling is based on the temp of the air traveling through the radiator and the a/c condenser not the outside temp so much. The start up high rev is just how these clutches work in ford fans. It sounds like it is operating correctly.

ther are two small tubes that open and close the valve to let in the hot air' one is from right under the carb intake and goes to the right (looking at the engine) right behind the fuse box. if there's o hole in that tube the heat will not work. just change mine becase the tube dry-rotted