Rich Black in Color Photos

Taking an RGB photo and converting to CMYK via an ICC profile will pretty much always result in some colour in the black areas. 

It's VITAL to have the correct CMYK ICC profile for this conversion since it sets not only ink limits but also controls ink recipes (as media and printing methods change so do the recipes for inks to make up image colours) . The printer or their agents should be able to give you this info. Guessing a CMYK profile is not a good idea. There IS NO GENERIC all purpose for CMYK profile. 

In order to stick within the ink limits of the print process in question it's also important not to do tonal edits after the RGB>CMYK conversion unless you have a high level of expertise. 

I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

I can't really see how a photo can have 100K black and not rich black, unless you deliberately made it so by editing the CMYK channels individually.

100K black prints on the black plate only, and usually overprints the other inks. It is normally used for text and graphic elements in a design. You can do that in InDesign, by defining a swatch as either, but not easily in Photoshop.

Rich black is all four inks, each at a certain % coverage that adds up to the total ink limit in the CMYK profile. Typical CMYK numbers will be someting like 78-68-58-94 or around that (those numbers are for ISO Coated 300% eci). That's the deepest black you can get in that CMYK profile.

If you convert an RGB file to any given CMYK profile, you get rich black. You do not get 100K black. A profile conversion will always result in rich black.

The visual appearance of 100K black is much weaker (grayer) than rich black. That stands to reason, since it uses only one ink as opposed to all four.