If you have a PRO-HD:
In the desktop player, go into Setup Assistant and select Picture & Audio Settings. Adjust Brightness and Contrast to your liking, click Apply. On the next screen, click Save.
I don't know whether this works the same on a Mac. If you have trouble, try from a Windows PC / boot your Mac to Windows / run Windows in Parallels, etc.
If you have a Slingbox 500 (the section you posted in), you are limited to whatever controls your Mac or TV has for brightness and contrast.
Thank you for using the Slingbox Answers Forum. I'm apologize for the inconvenience of not being able to really see the video when watching.
The above reply is a great suggestion! In addition, it also could be a form of selectable output control (SOC). This is only the case if you are using component cables from your cable box to your Slingbox. Also, it only happens if you have an HDMI cable connected to the same set-top box as the Slingbox. A work around is to disconnect HDMI.
Below is a link to an article that explains SOC in better detail:
Hope this helps!
The Sling Moderation Team