3.4L engines were the replacement for the 2.8L (and 3.1L). These will be a stright swap, even using the same motor mounts. These were used in GM F-bodies. 4.3L are another obvious bolt in swap.

The 3.8L was also available in the F-body. How much work it would take for the motor mounts is anyones guess. 3.8L engines were also available in GM G-bodies (Cutlass Supreme, Monte Carlo, Regal, Grand Prix, etc.) until 1987 and 1988. Newer engines have more power. If your Blazer is 4WD, I don't know how well the engine will bolt up to the existing transmission.