I've been using a load of 13.5 Blue Dot, 250 Hornady XTP HP, R-P brass, WLP primer, 1.610" OAL in my Blackhawks for a number of years. Seems like a nice, relatively mild load (never chrono'ed it). I'm not sure where I came up with that load, but it was quite a while ago.
Speer lists a maximum of 16.9 Blue Dot with their 240 JHP.

Is it possible this load is too hot for my Redhawk? The reason I'm asking is, when I was at the range last week, after the 20th shot, the cylinder became extremely difficult to release, and must be FORCED shut. Did I ruin something?

If 16.9 grains of Blue Dot is a maximum for a 240 grain bullet the 13.5 grains is going to be just fine for a 250 grain bullet. You haven't ruined your revolver but there is something going on. Maybe unburned powder or some other debris in a place I shouldn't be or some other mechanical issue.

Thanks Tim.
It was a small burr on the frame that was snagging the cylinder. Filed it off, smoothed it up and it's good to go!!