what causes blue smoke with a gmc 5.7 liter

You have a bad valve seals or guides

If it smokes when driving ther could be a ring problem. You could do a cylinder leak down test. Add compressed air to the cylinder with an adapter in the spark plug hole add air and li sin to where you hear air escaping. If its under the oil cap its rings. You can also replace the guide this way

Leaking out of the tail pipe-its an exhaust valve and out the intake/throttle body its an intake valve, oil cap or oil dipstick ring/s problem. Blue smoke is a sign of burning oil. If it goes away after a few minutes, it is typically due to it running past the valve guide seals into the cylinders. Once it has burned off, the oil runs down too slowly while the engine is running to be easily visible.

If it smokes all the time and worse after the engine is warmed up, that is typically due to oil sneaking past the piston rings. That will be worse when the oil is hot because it is thinner.

Also check the PCV valve. If it is plugged, excessive pressure will build up below the pistons and help push the oil on the cylinder walls past the rings.