why do the digital dashboard on the 1990 chey blazer go off ?

First, check your fuses (the fuse box is on the lower left side of the dash). If the fuses are fine then you have a wiring issue, most likely loose wires in the back of the dash.

My 94 s10 blazer had the same digital dashboard problem. The power module inside the dash cluster fails causing the speedometer or the indicator lights to go out. When the unit cools off it comes back on. A speedometer shop can remove the dash unit and replace the power module.