Does your particular Slingbox have pass through outputs? You can try to connect it to your TV to ensure that's the signal its getting from your DISH set-top box. You can also try connecting the output from the set-top box directly to your TV to make sure that's the video signal its receiving.
The most common occurance of this symptom is that there's a conflict with the HDMI output and the analog output of the set-top box. Unfortunately, there's no standard across manufacturers on how the outputs work together. Sometimes they shut off the analog outputs when the HDMI port is used. You'll want to verify the signal output from the set-top box that being sent to the Slingbox since it only can rebroadcast what it receives.
Its also helpful to elaborate on the model of the Slingbox and DISH box as well as version of Slingplayer. Just in case others have similar experiences with specific combinations.