Expression Blend SDK for SL4

ここです。
http://www.microsoft.com/downloads/details.aspx?familyid=D197F51A-DE07-4EDF-9CBA-1F1B4A22110D&displaylang=en



ちなみに WPF は以下。
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=75e13d71-7c53-4382-9592-6c07c6a00207