It all depends of the way you apply your OD. If you want to keep the darker area you have to apply light coat of thinned OD with an airbrush. A spray can of painting it with a brush would be to thick and the black would not go through the OD.
See the way I painted my Churchill, I did not use black but my base coat (SCC15) + black to get it darker. I did the preshading with that color then I applied thin coat of my original SCC15 mix :
viewtopic.php?t=34159&start=60
You can see the darker area still show through the last coat.
An interesting video :
The guy did an experiment on a sherman, using pre shading on one half and black basing on the other half. Results are very similar :
https://www.youtube.com/watch?v=HxaPi545_T0