I have a older programmable thermostat with red, white, yellow, green my wire out of the wall are red, white, yellow, blue

Not every t'stat wire is used. Many HVAC companies install a cable with more wires than needed so you can change syatems. The blue wire is usually for a heat pump. What type of system do you have, is it running properly and if you are changing t'stats, where was the blue wire before or is it just loose?

The blue wire was used instead of green. I would say that the blue wire goes to "G." That wire controls the fan relay. "Y" is for the compressor, "R" is the power to the switches in T-stat, "W" is heat. This is for conventional ac/ electric or gas heat. In a heat pump you have an orange or blue wire for reversing valve, depending on where you live. DrTom4444