Chestnut is my best guess.

I was thinking about this some more, and agree that it could be black alder too. I also think that it's going to be important to remember that even within a group (buckwheat, chestnut, etc.), there can be tremendous differences. The only real way to copy M.F.K. Fisher would be to go back and get honey from the same person that she got if from. I think you're setting yourself up for a fun honey-tasting party!

Just to make things more confusing, it might also be black alder (bourdain) honey, which is also dark and a bit funky...however, I'm not sure how things were back in M.F.K. Fisher's Dijon days, but my bet would be buckwheat, just based on cost and the amount you use in pain d'epices. This is completely a guess based on my own narrow experience of food-shopping in France, but buckwheat has seemed like regular-old-everyday honey, and the chestnut honeys I've seen have been on the pricey side (cheaper in Spain, for some reason).

We went through this a little bit last summer for another recipe--in that case we wanted (and actually had in the cupboard!) lavender honey. You want something darker. There's some good info here:

http://www.frenchselections.com/deco/honey.mv

The gist is
Miel d'Acacia (acacia) is a favorite of cooks because it remains liquid for a long time and has a light aroma. Miel de Lavande (lavender) is very sought after because of its distinctive flavor. Miel de Romarin (rosemary) is a classic that was enjoyed by Greeks and Romans. Miel de Sapin (fir tree) is syrupy, dark in color with a faint scent of resin. Miel de Châtaignier (chestnut) seduces many because of its bittersweet characteristics. Miel de Framboisier (raspberry) is white and has a very subtle flavor. Miel d'Eucalyptus (eucalyptus) is a true original: because of its slightly pharmaceutical after-taste, it is best used to flavor a winter grog...

Wow - I have no idea but am very impressed with your level of research here!