Try the following fix:
Create a bat file with this information (Expression Blend 4 Fix.bat):
cd %windir%\Microsoft.NET\Framework\v4.0.30319
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend 4\Microsoft.Expression.Framework.dll"
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend 4\Microsoft.Expression.Blend.dll"
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend 4\Microsoft.Expression.Project.dll"
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend 4\Microsoft.Expression.WindowsPhone.dll"
Create a bat file with this information (Expression Blend 5 Fix.bat):
cd %windir%\Microsoft.NET\Framework\v4.0.30319
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend Preview for Silverlight 5\Microsoft.Expression.Framework.dll"
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend Preview for Silverlight 5\Microsoft.Expression.Blend.dll"
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend Preview for Silverlight 5\Microsoft.Expression.Project.dll"
ngen uninstall "%ProgramFiles(x86)%\Microsoft Expression\Blend Preview for Silverlight 5\Microsoft.Expression.WindowsPhone.dll"
Run the bat file as a Administrator
Or Download it from my SkyDrive