Microsoft has released an updated version of the Expression Blend Preview for Silverlight 5 which provides support for creating Silverlight 5 and Silverlight 5 SketchFlow projects.
You may probably recollect that Expression Blend Preview for SL 5 was released in September 2011. This update comes with a GoLive license and its ration date has been extended to June 30, 2013. Moreover the Silverlight 5 SDK installed with Blend supports the RTM version of Silverlight 5.
Note: Expression Blend Preview for Silverlight 5 can be installed side-by-side with Blend 4. Blend Preview for Silverlight 5 supports only Silverlight 5 projects. For Silverlight 3 and 4 projects, please use Expression Blend 4.
You can download this release from the Microsoft Download Center: Microsoft Blend Preview for Silverlight 5.