First off I want to welcome you to our online answers forums! Just so I am clear with your setup, you stated that you have your set top box hooked into your VCR and then your VCR hooked up to the Slingbox and Slingbox to TV? We do recommend that you hook it up as follows... Set top box output into the Slingbox input and then the Slingbox output into your VCR and then the VCR output into your TV. However, if this setup has worked for you before, then the first thing I want you to try is hitting your TV/VCR button your VCR. It may have automatically went into VCR only mode when the power was out, which would explain your "blue screen." If that does not resolve your issue, you may want to try a new set of cables.
However, if these troubleshooting steps do not work, please reply back and we will be more than happy to further troubleshoot this issue for you.
Sling Moderation Team