I suspect there's an simple answer to this, but is there a easy way to search for b&w photos within a selection?
Whats a b&w photo?
An image where all 3 or 6 color changes always have the same value for a given pixel? Or about the same?
A monochrome image?
A color photo that has been de-saturated in some way by applying a filter?
An image which uses only black and white pixels or a color palette (clip art)?
Maybe an image where the ICC profile is set to gray?
ICC_Profile::Header\16\ColorSpaceData: GRAY
A file converted to grayscale with Lr has the tag XMP::crs\ConvertToGrayscale set to true.
You can search for these in the Filter panel.
There is a discussion about why this is not easy here: https://www.photools.com/community/index.php/topic,11867.msg84624.html#msg84624/topic,11867.msg84624.html#msg84624