Gumbo base tastes burnt, but the final product does not
I was born and raised in the heart of Cajun country.  My entire family loves gumbo, especially my moms.  This is what I learned:  get the roux as dark as possible without burning it (that makes the best gumbo).  I think that's what happened to yours; you probably got it just right. Then you add the other ingredients so that it mellows down the bitter taste.  I, unlike my mom, have actually slightly burned my roux many times. When I do this I add half a potato for about 10-15 min and it helps remove the burned flavor (don't forget to take out the potato). Maybe there is as yet no rhyme or reason to why the slightly burned/bitter turns into deliciousness after its cooked with other ingredients for a while, but I thank heaven for such a wonderful mystery ;)