Master Replicas' put scope reticle decals on theirs: red lines with black background. I don't think they were canon, but many people have mimicked them for their blaster builds.
WizardOfFlight made these for printing on transparencies:
http://wizardofflight.deviantart.com/art/Scope-Reticles-441963399
The top/right matches the one used on MR's "Han ESB" blaster's M19 scope replica.
The bottom/left matches the one used on MR's E-11 blaster with a M38 scope replica.
You could find more examples if you would look around more, including blends of the two styles and images that are black and red like MR's original.
Here are a few with reticles over images.
Here is one more that I made for an E-11:
I think it would be cool to have a lenticular lens inside with two frames from a movie. That way the image would change when you moved your head and/or blaster. I have not seen anyone do that.
Another idea is to light up the transparency: red in "blast" mode, and blue when the blaster is in "stun" setting. I have seen red but not blue.
The screen-used blasters did not have anything fancy that I know of. The real scopes had lenses, of course.
The blasters that had been cast in resin had circular (bicycle) reflectors glued to the back of the scopes.
