It looks like you can book IAD-BLF-BKW as separate flight #s if you don't mind waiting in BLF for a couple of hours (shudders!) for an extra 500 miles I've had various travel sites spit it out as such when I've been pricing a trip to visit family who live equidistant from BLF and BKW.

I know the ORF-PHF flight is valid in a WAS-PHF routing and has a separate flight #, giving you something like DCA-PHL-ORF-PHF and PHF-PHL-DCA for hardly any flight miles but 2500 credited miles--and for reasonably cheap for that sort of flight, too (saw $180 for this weekend earlier, which is what brought up my question on weird routes to begin with)