Is it only with the blinker?

Try a few things.

Try flashing the headlights on and off, and see if it does the same.

Try mashing the brake pedal and looking for the RPM change.

If these other amp loads do it, you could be looking at a ground issue.

Just for good measure I would remove and clean the battery terminals, and where the ground strap goes to the block. Then give it a try.

If it is only doing it on blinkers, then back track your grounds to the blinker.

If all else fails, you may just need a tach filter replaced or installed.