can't create xc:black ppm in truecolor

[magick]

If you look at the first few lines of black.ppm does it say P6? That indicates the image is truecolor. If not, try ImageMagick 6.2.9, the latest release. It works for us. You can also try setting the output file to pnm:black.ppm.

[glennrp]

but all I get is Type: Bilevel, Colorspace: Gray and Channel depth: Gray: 1-bits...

"identify" analyzes the image not the file. Even if your image is PPM, "identify" will look at the pixels and see that they are all black or white,
and then report what you saw. If you really want to know what is in the file, use a bit-dumper like "od -c file.ppm" and (as mentioned above) look for the string "P6" at the beginning of the file.

Glenn