As others have written, your fat/flour ratio is off. At most, they should be equal amounts, with the fat part always being equal to or higher in the ratio, and the fat should be in liquid form before the flour is added. Then you have to stir, stir, stir as soon as you add the flour until a smooth mixture forms. Then, you keep stirring (or whisking) until the roux gets to the color you want. Add the warm to hot liquid in small amounts stirring constantly until the roux is thinned out, then you can add in the rest of the liquid. Doing it that way keeps lumps from forming.
BTW, the vast majority of Cajun roux (as opposed to French roux) are made with oil, not butter. Butter is far more likely to burn as you get into the darker roux than oil, which will give you a funky, burnt taste.
If you want a butter taste, either use margarine or a combo of butter and oil so the burning point of butter will be higher. For a gravy, I would not take a roux containing butter darker than a light medium brown.
Oh, are you using homemade or purchased stock? Some commercial stocks have off tastes because of the preservatives and other chemicals used in them, so make sure the stock you are using, if purchased, tastes okay to you.
Also, be aware that the darker the roux, the less thickening ability it will have.
