Could be a freez out plug you need to find out where it's coming from and see if there is a plug there. If so then you need to replace this plug.
There is a heater hose that goes to the left rear top side of the intake manifold and that fitting leaks engine coolant sometimes and it will run down the back of the intake around the distributor. If that fitting is not leaking then the intake manifold gasket is leaking coolant in the corners of it. REplace the intake gasket. That's all that can leak coolant on that year engine. If coolant is leaking out between the transmission and engine block then you have a freeze plug leaking in the back of the engine, between the transmission and block. Must remove the transmission and flywheel to fix it.
it might be the intake gasket
The engine block coolant drain plug on 4.3 liter Chevy Blazer is located on the bottom of the engine, near the oil filter. It allows coolant to be completely drained from the block during maintenance.
If you are sure that it did not freeze and crack the block, Then the intake manifold gasket is leaking. That is a common problem with the vortec Chevy engines. You need to replace the gasket with a factory Chevy gasket from the dealer.
The head gasket sits between the head and the engine block if it is leaking out where you can see the coolant, yes you can smell the coolant if it is leaking into the engine no you cannot smell the coolant. You can check the oil to see if it has coolant in the oil if it does then the head gasket might be blown.
The engine block coolant drain plug on a 350 Chevy Tahoe is above the oil pan, mid engine on driver's side
That is a common problem with the vortec 350 engines. The intake manifold gasket starts leaking in the corners of it. That is where the water jackets are and it's only about 1/4 of an inch wide and when it starts leaking it allows engine coolant to leak into the lifter gallery and into the oil. Need to replace intake gasket as long as you no it did not freeze and crack the engine block.
Freeze plugs. More likely that hoses are leaking or thermostat housing is leaking. Freeze Plugs generally NEVER leaks unless engine block has had frozen coolant in it. Hence, Freeze Plugs.
Either a blown head gasket or a cracked block possibly
I would doubt you would pour engine block sealer into coolant. More likely it would just need to be put in with the oil. Like lucas. I dont know your particular product. Depends also, are you leaking coolant, or burning it?
Check the freeze plugs. large round plugs on either side of the block. Those are there for if your coolant freezes, they will go instead of cracking the block.
Rear of the engine block, near the oil pan.