It looks like you have typos in your XAML:
Alubm
instead of Album
and BlentTest
instead of BlendTest
I'm guessing the errors are actually true compiler errors, and correcting the two typos above will most likely "fix" the designer.
Edit:
The most likely candidate that I see is that your converter is in a separate project from your XAML file (but in the same solution). If that's the case, make sure to specify the assembly in the XAML declaration, and make sure the other project (with BlendTest) is referenced correctly. ie:
xmlns:local="clr-namespace:BlendTest;assembly:BlendTest"
If your referring to a type (with namespace) defined in a different project, the assembly reference needs to exist, as well.