I've never had to enter the service menus as yet, so haven't even seen the screen you'd be in.
My previous experience of Gamma correction was on a Pioneer DVD player that could memorise custom settings for individual discs. The original DVD release of
Dune had very grey blacks - as if the brightness had been turned up, but only on the blacks.
I reset the Gamma settings for just this disc, so that any time I inserted it the player would make the adjustments, and it worked a treat. It was quite easy to do. In simple terms it allowed me to darken the blacks and nothing else.
However this tweak was in the DVD player and was customied for one disc. If you alter the settings on the Sammy it will be for the screen and everything you watch. I shouldn't imagine there would be much to worry about if you make a note of the original settings before you start adjusting, so if you don't like what you see then you can switch back.
Don't know if the Helpkine would give advice as we're not really supposed to know about the service menus. No harm in asking.
I have the Samsung 46" and the black levels are superb and the SP50s I've demoed were just as good. There is the occasional disc where this is not the case, but on others there are blacks so deep it's like a total absence of light, so you will get variations from disc to disc. Make sure it's not just the material you're watching
Try this for a test. Check out immeditely after the opening credits of
Indiana Jones and the Temple of Doom, where Indy comes and sits at the table with the Chinese gangsters. He's wearing a white tux and a black waistcoat and black pants. That is the blackest black I have ever seen. You'll be in doubt of what I mean if your settings are correct.