I have no reverse on my 1998 chevey blazer 2x2

Before guessing at transmission problems, check the fluid level (car warmed up, engine running in Park on level ground) and have the PCM scanned for trouble codes FIRST.

Secondly, put a line pressure gauge (or visit a local trans shop) on the pressure test port and go for a drive to verify proper line pressure. Base pressure at idle in P,N,OD should be around 70psi and around 140psi in R. Max line pressure should be at least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump is worn out. If max pressure is low, check EPC solenoid. If reverse is slow to engage, and line pressure does not come up to at least 130psi in reverse, replace the boost valve with a 0.490 or 0.500" boost valve (easy fix in pan).

Slow, slipping or no reverse: “lo-reverse” clutches are worn out, fluid leak in the reverse apply circuit, or broken sunshell. It is possible to remedy a fluid problem by removing the checkball from its cage in the case in the rear of the trans may help (must remove valvebody), or adding a high-viscosity additive such as Lucas Transmission additive or other seal restorer product. May also have worn boost valve (can replace in the pan).

Good luck and this should be a easy fix unless it's your sunshell that has gone bad, and call around for a transmission shop you can trust.