Blend is not part of Visual Studio; Expression Studio is a separate suite. VS2010 does include a basic design canvas for WPF / Silverlight, which might be adequate for some business-style applications (e.g. conservative look and feel, simple layout), but it's pretty limiting if you want to make a visually compelling RIA.
Likewise, Visual Studio is a far superior coding / debugging environment, although Blend does have a rudimentary source code editor for VB / C#. If you have both products, they are loosely integrated via "Open in [product]" options on the context menus. Blend can also leverage other editors through a generic "Edit Externally" right-click option (based on Windows file associations).
Right now I am trying out the free Visual Studio Express together with the 60-day trial of Expression Studio 4 Ultimate (which includes Blend) on a new system. After I installed Blend, I reran the Silverlight API setup and got two new options to install the Esri templates in both Expression Blend and Visual Web Developer. (As for whether the templates work in both environments - well, on the VS side I am still plugging away, trying to sort out some broken dependencies. Meanwhile, on the Blend side everything just worked.)
