A code editor and a UML diagramming tool are two separate tools. Sometimes, they are nicely integrated into a so-called two-way tool, where the source code and UML diagram are logically connected and any edit in one representation updates the other.
One successful example is
Together ControlCenter. Unfortunately, Together is wildly expensive (it's targetted for companies) and it also requires a lot of system resources. They have a community edition available which only does class diagrams, but currently you can't really get it. They just released version 6.0, but the community license you can download only works w/ 5.5.
Together ControlCenter, as the name might suggest, is a very ambitious tool. In this sense, it easily rivals VStudio in features. Together can call into an external editor (i.e. emacs) if you're disappointed with their native editor. Together also supports C++ and a couple .Net languages, but it's primarily a Java development platform.
The other example that comes to mind is IBM's
Eclipse IDE.
Rational, which is the de facto leader in UML technologies, has a product called Rational XDE that plugs into Eclipse. I have no experience with this.
Installing VStudio shouldn't break a Java tool, but it's possible. Why can't you re-install BlueJ?