Is it possible to comment out blocks of code that contain comments?

In SQL Developer, I highlight all lines of PL/SQL that I want commented and use Ctrl + /.

Obviously, you would like a way to comment and uncomment multiple lines quickly. This will put -- in front of each line you have highlighted. Do the same command to uncomment.

Shortcut to comment a line: Command + Option + / (in Mac)

you can assign a function key through Tools > Preferences > Key Configuration (Edit / Selection / Comment Lines).