My 94' had suffered the same problem as alot of 1st gen. blazers do with the vacuum actuators not allowing air to blow from the dash vents, and only to the floor or the defrost position. Sooo I took apart the lower cover under the glove box (which is just screws mainly) and found the blend door that was failing to work- It has a hinge-like mechanism that when manipulated actually can close that blend door and allow air to blow 100% out of the dash! Heres the funny part, I took a large enough random screw and placed it in between the hinge on this mechanism to allow the door to stay closed and completly cut off air flow to the floor. Now I get great dash airflow, and great defrost airflow. If you wanna compromise and do without the floor, and get airflow restored back to the dash without spending an arm and a leg this works great! Just an fyi.

 

Whenever I fixed mine I screwed a hacksaw blade the the actuator and left it sticking out towards the right.

No whenever I want defrost or AC I move it towards the left.

Whenever I want floor heat I move it towards the right.

This way you have the best of both worlds and not just one, Try it I bet you like it!