Please be careful! I've heard about Flitz being used and even recommended, but when I used it on one of my guns I took off a little more bluing than I should've. It didn't wreck the gun, but there's an area where you can see the finish is thinner in certain light. And I used a small amount with not really intense pressure for not very long.
Do this first - either find a beater blued Rossi or Charter Arms with lots of wear and prctice with the Flitz on that...or take the grips off the Ruger and practice on a spot covered up by the grips. Squeeze out half the amount of Flitz you think you'll need for the job, then only use half of that. Oil and water might work for diluting.
Before flitz, You may try mineral spirits. but try on an inconspicuous area first. My Colt didn't like MS, but some people have had luck. Thankfully I tried it on an inconspicuous area of the Colt, and hoppes 9 took away the ill effects.