out of nowhere my engine started backfiring while running, it happened after i put some gas and fuel system cleaner in it why is this happening

 

My guess is that you had some bad gas and that it having sat for a while untreated could cause it to burn improperly. I would burn this tank up and then fill up with some new gas. Also depending on the mileage and age, you may need to start doing some tune up items. These would be sparkplugs, wires, distributor cap, rotor, clean the carb and other sensors, replace the fuel filter (this may be a big cause of your problem), and make sure the other filters are good. I am not too familiar with the older k5 motors so I am not sure what they have on them, ie IAC valve, EGR valve, MAF sensor, etc. If you have these make sure they are clean and operating properly.

Also if you could add a signature, under the my profile edit function, and add your make, model, engine, drivetrain, year, maybe mileage etc. (general info about your vehicle). This way we have a better idea of what could be wrong with your blazer. Also if you can be more descriptive about what led up to the problem, and what exactly it is doing (noises, what its idling at, does it do it when accelerating, etc.) Just wanted to let you know for future problems and to help in this diagonosis.