My eyes are not what they used to be, apparently. I've been having trouble cutting very dark fabric, especially when I'm cutting on the whole inch mark. I use my 5-inch square Omnigrid ruler a lot, but it can be really hard to line up the skinny black whole inch line on black fabric.
So, I came up with a solution that works on lots (but not all) black or really dark fabric:
Just turn the fabric over and cut on the wrong side, which is likely to be quite a bit lighter than the "right" side! Sooooo much easier to see. Like I said, it won't work for all black and dark fabrics - if your fabric is yarn dyed, like a homespun, you're probably out of luck.
I'm probably 100 years late in discovering this trick, but I thought I'd pass it along anyway.