Keying not working with black?

If you have black and white footage intended for use as a matte the best option is to use Track Matte. Just put it above the original footage and choose Luma Track Matte from the options. Please read up on using a track matte by typing "track matte" in the search help field at the top right corner of After Effects. This is a common production technique that you should fully understand.  Blend modes are not suitable for keying. The only thing they would do is darken or lighten the layer below depending on the mode you choose. Blend modes are useful for adding things like glows to footage because you can easily the black in something like a lens flair applied to a solid and see only the lighter colors by choosing Add or Screen for blend modes.

You should also read up on blend modes and experiment with them. Almost every composite that I work on uses both blend modes and track mattes.