How to mount handrail brackets to a concrete block wall?
I'd go with hex head tap cons.  Pre drill the holes.  If you are really worried then get some epoxy and put this into the hole before fastening - this is how rebar is inserted after the fact in structural applications.

If you want it to look perfect then probably choose whatever fastener look bests based on the bracket and drill a hole smaller than the fastener, ensure you blow the dust out of it and put structural epoxy into the hole.  For ~$150 you can get the two part caulk gun and the epoxy.  Something like this : 

https://www.dhcsupplies.com/store/p/2464-SET-XP-22oz-Epoxy-Tie-Adhesive-with-Nozzle.html
https://www.dhcsupplies.com/store/p/468-EDT22S-22oz-Manual-Epoxy-Dispensing-Tool.html