I've never used black ash. If it were red oak, it may turn black and no way to get it out.
No idea of the stain you are using. The amish like to use fruit wood stain on their oak, which I like to use too. I don't care how much stain you put on oak, in time, it will diffuse and turn lighter.
As for putting poly on wood. I spray my poly with 10-20% mineral spirits. Spray the stain too. Making a Christmas present, snow on the ground, temp around 30. Took the project outside in the cold and sprayed it. As long as the sun is shinning, it will flash dry in less than 30 minutes.
Pic of king size head board and the snow.