Indesign problem

A gradient as a spot of black?

You mean a black to white where you specify the black as a spot? You would need to do a gradient of the black spot (100% tint) to the black spot (0% tint).

Alternatively, you can do a spot to CMYK gradient in Indesign and it will separate onto different plates just fine, there's lots about this online, a brief summary:

You can create a gradient between two spot colors. However, to avoid moire patterns in the gradient, you must assign different screen angles to those spot colors in the Inks dialog box.
For instance, if you had a gradient of spot red to spot yellow, you would want to make sure each color had a screen angle that is 45 degrees differnt from the other. Ask you print shop for details on setting the screen angles for spot colors.
A gradient that contains both spot and process colors will separated onto both the spot and process color plates.