Many black ribbons used a plastic gear mechanism for advancing the ribbon. This mechanism didn't deteriorate like the felt rollers inside the color ribbons. In making these new ones I have specifically asked the factory to make them with the gear drive instead of felt rollers so they will last longer in storage. Since I had a say in manufacturing process I figured it would be stupid to repeat a design that we now know was a bad idea.
It is possible to have them make black ones as well. They actually offered a color choice for the single-color ones and if needed can make the ribbons print in a color other than black. That seems like it would be kinda cool but not worth spending the money do have different colors made at this point.
You are correct that ribbons are still being manufactured for impact printers although I don't think they are being made specifically for ImageWriters anymore. Having said that, you can buy new black ribbons for these printers on eBay for a couple dollars a piece. They are so cheap that it doesn't make financial sense to have them manufactured again until the supply is exhausted (which is the case with the color ones.)
There are three reasons (that I know of) for black ribbons still being easy to find:
1. Black ribbons were more widely distributed, manufactured in MUCH greater numbers and had much higher demand. Nearly every office supply store in the 1990s was guaranteed to have black ImageWriter ribbons. That's a big supply. Color ribbons were pretty expensive when they were new and they were a niche product even then. On a recent episode of the RetroMacCast John talked about how he was the color re-inking guy for his local computer club. The color ribbons were so expensive that they would bring their spent ribbons to him to be re-inked rather than buy new ones.
2. Apple never officially released a color driver for the ImageWriter II. Only Apple II users could natively print in color on the IW II and late in the life of the printer Apple IIs were fading and Macs were on the rise. Yes, there is a third party driver but at the time you had to pay for it and most people didn't. Chances are, if you went to buy a ribbon for an IW II, it was a black one.
3. For some reason many of the black ribbons used the gear drive instead of the felt rollers. This makes them more reliable and less susceptible to failure over time. Therefore, if you find a stack of black ribbons that are still sealed, they will probably still work like new. (This is not the case for color.) I have no idea why these used gears and the color used rollers. I'd love to hear from an engineer to figure out why there is this design difference.