At this point this is not possible because of the way the modal overlay is built. In order to achieve semi-transparency in IE, it uses CSS filter. If such filter (which is an ActiveX object) is combined with animation of any kind, it will result in a black background. That is why we prefer to apply the modal background at the end of the animation sequence.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items