–Is there a formula that gives an intrinsic B-V index by stellar temperature?
–Is there also a constant for the ISM that can be multiplied by distance to determine reddening?
If so, couldn't you combine the two and compare the results with actual B-V values?
–Stars whose B-V values match the calculation could be considered a "clear view" through the ISM.
–Single star anomalies might indicate an atmosphere or nearby shell that obscures the light.
–Groups of stars in the same area with a similar B-V shift could indicate nebulosity and provide a rough distance.
With a list of stellar temperatures and distances (down to mag ~6) in excel, it wouldn't take long to find the outliers; maybe a little more complicated to plot them.
Perhaps it would be interesting for areas away from the galactic equator? I’ve oversimplified things, but crudely tracing nebulae based on data seems like a nice preparation for contemplative observing.