There are a few ways of dealing with the filmic rgb module. The way you mention is one and the other is the way I approach it.
I did get the impression that you didn’t and wanted to point it out to you as an alternative. As you can see in the image I posted I do not have any problems with the blacks and I am all but certain that this is due to the 18.45% and use exposure approach.
Aurélien mentions setting middle grey to 18% many times in his replies. Here are 2 of them: one, two
Others also started using this method of working with filmic rgb. Boris Hajdukovic, starter of this topic, is one of them. This editing moment and the few replies after it might be worth your time.
And there is this topic about Filmic RGB defaults
Don’t get me wrong: I’m not implying that this is the way of working with filmic, it is just one that seems to make the most sense to some (more and more?) of us.
In the end it is all about finding a way of doing things that works for you and gives you good results.