If you don’t have Tim Pope’s unimpaired.vim, you should. Unimpaired adds many useful pairs of mappings including one for adding an empty line above or below your current line.

In normal mode, use [<Space> and ]<Space> to add an empty line either above or below your current line. With your cursor on the def bar line below, tap [<Space> and a line will be effortlessly inserted above while your cursor remains on its line.

def foo
end
def bar
end
def foo
end

def bar
end