You can disable the artifacting from MESS user interface. It's under Options-Configuration menu.
No, no, no... The dark green background is not a M.E.S.S. artifacting... It's what happens in the REAL machine, even though MC6847's specification says it should be fully black. I wonder if it is a particular MC-1000 "glitch", or if it is the standard functioning of MC6847, in which case the emulation should be changed accordingly.
That's why I asked for people who have a real MC6847 based machine to test it, if possible.
(I wonder if I should turn these posts into a separate thread, so that more people will see it.)
(Edit: I'll do it.)