OpenGL function glGetActiveUniformBlockName()
is documented here. The short description reads: [it] retrieves the name of the active uniform block at uniformBlockIndex within program.
How can I make an uniform block active?
A uniform block is active in the same way that a uniform is active: you use it in some series of expressions that yields an output. If you want a uniform block to be active, you need to actually do something with one of its members that materially affects the outputs of a shader.