The rule of thumb - revisited
All wind instruments (including the blues harp) operate with feedback: see my essay What makes reeds oscillate (Part 1). The rule of thumb is, to be more precise: The frequency of an oscillating single reed in a feedback circuit lies below (above) its eigen frequency, if it is a closing (opening) reed.
In the blues harmonica two reeds oscillate in one channel, and that makes things more complicated...
We will only look at channels #1 to #6 (for the upper channels you would have to interchange "blow" and "draw"). With normal blow notes, the closing reed oscillates primarily. If we assume that only the closing reed is responsible for the feedback, we can apply the rule of thumb to one reed, and the playing frequency is below the eigen frequency of the closing reed. The opening reed is also excited by the pressure fluctuations to an oscillation (with this playing frequency), but it is not involved in the feedback process. Therefore, the rule of thumb does not apply to the draw reed, which is forced to oscillate below its eigen frequency. As we will see in another essay, it fits that the reeds oscillate almost antiparallel with blow notes (both reeds oscillate almost simultaneously outwards or simultaneously inwards). Similar to normal blow notes you can argue for overblows: The draw reed provides the feedback, the blow reed is forced to resonate.
During bending, both reeds oscillate in a common feedback circuit: The pressure fluctuations in the chamber influence both tongues, and both tongues influence the pressure fluctuations. One (unpleasant) example of feedback is the dreaded feedback whistling on stage: the sound from the loudspeaker influences the microphone, the microphone influences the sound from the loudspeaker. Just like feedback whistling on stage, the pressure fluctuations and tongue vibrations in the blues harp "agree" on a common frequency. By oscillating almost parallel in bending, both reeds together build up the feedback process and keep it alive (as we will see in the essay to be written) - they "pull on the same rope". With normal draw notes (on the lower channels) both reeds also oscillate almost parallel. Whether only one tongue or both tongues are involved in the feedback process will probably not be possible to separate. The fact that the transition to draw bends is continuous rather speaks for the second view.
One final remark: Of course, the blow reed is called this way because it is the one that oscillates in normal blow notes. But why does the blow reed win? Why doesn't the draw reed oscillate when blowing with a relaxed vocal tract? It is important to know that the feedback between tongue oscillation and channel pressure fluctuation only works under certain additional conditions. And these additional conditions are only given for the respective closing reed when blowing or drawing in a relaxed manner. A separate essay is planned on this topic (What makes the reeds oscillate - Part 2).