Interesting - thanks for the info. You're right, I am running 64-bit Windows 7. I also found this similar bug report
Our main project with the chart is a fairly large app, and I haven't had time to cut it down to a minimal example that I can send to you. In our main project, the chart shows up correctly in the VS 2010 designer, but not in Blend. There are a number of other Telerik controls that Blend has issues with in our main project - see my comment about this
in a separate thread. Blend gives us different errors for those controls than it does for the chart - so it may be a separate problem.
I did create a sample project from scratch, to test the chart issue. In this sample project, the chart shows correctly in the VS designer, but only if the project has been built. If I clean the solution, the sample project gives me the same error in VS that I get in Blend. But strangely, if I clean the solution in our main project, that chart shows up fine in VS. So the sample does not exactly reproduce what's happening with our main project.
I'll submit a support ticket with the sample, in case it's useful.