Blend and VS come close to each other but both have their own audience. Underneath both run on the same engine. Visual studio is aimed at developers while Blend is aimed at UI designers. That doesn't mean you can't achieve a lot of the functionality of Blend in VS and vice versa, it's just not optimized for it. Designing animation, storyboards, attaching behaviors, ... is all easier in Blend. Other development features are easier in VS (debugging, server connections, source control, managing non UI sourcecode, ...). Both are very much connected, you can even switch between them.
WPF Tooling in Blend and Visual Studio 2015
Blend and Visual Studio - Why Two Tools?
Designing XAML in Visual Studio