It seems that you are having some issues with your Slingbox PRO-HD video signal since it is too dark.
First, I recommend you to run the AV configuration one more time. This process will adjust the video settings automatically. Go to setup.slingbox.com and select Configure Inputs.
If the issue persists, you will need to download and install the SlingPlayer software version on your computer. This application will allow you to adjust the settings in a manual way. You can find the SlingPlayer software on our website at www.slingbox.com in the "Downloads" section.
Hope this helps!