2000 blazer cold start problem
 

Out of nowhere the blazer won't cold start.It cranks over but no fire it seems.It will cold start fine for over a month then act up one day.Smoke will come out the air duct and the car seems to diesel after you turn the key off and my friend says it was turning backwards.Any clues will be appreciated,Thanx

Are you getting spark when this happens? Should be a nice bright blue spark.
How old are your plugs, wires, cap and rotor and fuel filter?

If those are all good then you will need to run a fuel pressure test as well as a leak down test to check for fuel delivery problems.