If your black & white treatments are only done in Aperture using the Black and White brick, you can create a smart album with an Adjustments rule. Set it to “includes” and choose “Black & White” from the pick list.

However, this isn’t a general solution to finding *all* black & white photos. For example, if you remove color by dialing down saturation, applying a gray color monochrome, or use a plugin like Silver Efex or OnOne Perfect B&W … such photos won’t be caught by the smart album.

HTH,
-scott
http://sdaven.blogspot.com

Then I think the keyword route is your best bet. I do the same thing … keyword photos with any plug-ins used on them.

Would an AppleScript be of use here? Instead of directly invoking the plug-in, invoke a script that:
- determines what plug-ins are installed
- present you a list
- launch the plugin
- when returned to Aperture, apply a keyword with the plug-in name to the photo

I’ve waffled on actually writing the thing. The script would avoid having to remember to apply the keyword. But it would be more cumbersome to launch (a right click on the image is about as easy as it gets :). And not bulletproof for suites like OnOne, where you can bounce through several plug-ins before finishing the roundtrip to Aperture.

It would be nice if the plug-ins offered a preference to apply a keyword on the resulting image. I recall the Photomatix plug-in has that, but haven’t seen it in other plug-ins.

-scott
http://sdaven.blogspot.com