You'd see the same thing in other terminal emulators, i.e. this isn't
a mintty issue.
Ctrl-Z suspends vim and drops you back to the invoking shell, but when
you invoke vim directly from the terminal there is no shell. Hence the
blank screen, and no command line to issue the fg command. What you
could do once you are in that situation is to open another window with
a shell and send a continue signal to the vim process, using 'kill -
CONT'.
If you do need a shell while in vim, you could use the :sh command to
invoke a new shell instead of trying to drop back to the non-existent
invoking shell.
Andy