Again I didn't say it couldn't be done and I didn't say it had to be BB as I recommended a sand to the OP. The recommendations are to give a person the best chance at success. Obviously you have run into troubles during your process and that is what the recommendations are trying to avoid. Fortunately you were able to endure and end up with nice fish. You right water quality is of the utmost importance but substrate has a direct impact on that. Yes there are very successful discus keepers with planted tanks.
I had mine in a tank with black gravel also. Personally I was not happy with the appearance of my fish. I didn't like my fish being darker than they should be and the peppering of the PBs. Some may be but I was not. Changing to white sand made a night and day difference in the appearance of my fish. They all displayed full vibrant colors.
I am only speaking from personal experience and trying to give the best advice I can.