1999 Chevy Blazer white smoke

White smoke indicates either water vapors that have just set up in the exhaust system OR coolant leaking through the combustion chambers and burning out the exhaust. Check your coolant level and oil level. If both are good and fluids are of normaly appearance than it was probably just water vapor. If coolant is low or coolant has oil mixed in (or vise versa) than there is damage somewhere i.E. Cracked head gasket, cracked head etc.