You can get White on Black labels for the label makers. It might take a bit of trial and error with the font size and spaces, but it should look pretty good.
I tried it with my Brother P-touch label maker. (I printed size 12 with 3 spaces between each #). I don't have an 18M to put it on, but it lines up pretty close to the 18S Quick Plug slat that uses the same receptables.