Black gun = replica firearm which is illegal to import for sale purposes, and illegal to sell to individual. Of course, we know there are ways around that, but it's expensive.
Clear receiver = imitation firearm which is legal to import and sell. Manufacturers of mid-range black guns are now producing the identical guns, but with a clear plastic lower (or upper in some cases) for the Canadian market. Many don't like that, but it does open the door to more affordable guns. There's a nice, untapped potential market in Canada, and it's nice to see reputable manufacturers making higher quality stuff that's Canadian legal.