The oil pressure sending unit on a 1990 4.3L Chevy S-10 Blazer is located on the driver's side (left side) of the engine block, near the oil filter. It's a relatively small unit with one or two wires connected to it. You'll likely find it fairly low down on the block.
However, the exact location can vary slightly depending on the specific engine configuration. It's best to consult a repair manual specific to your 1990 4.3L S-10 Blazer for a diagram and precise location. A Haynes or Chilton manual would be helpful.
