1997 Chevy Blazer Air ducting problems

Likely cause is a bad vacuum line leading to the dash controls. The common break point is right at the plastic tree connector that is located between the brake booster and drivers side fender. The plastic cracks and you lose vacuum.
Default position in the event of vacuum loss on most heating systems is to switch to defrost mode.
It could also be a defective vacuum reservoir or line going to it. That is the ball mounted under the hood on the drivers side. If the lines on it are cruddy, flaking replace ALL the vacuum lines. Cheaper and easier than doing them one at a time.