Very interesting question. I have developed so many food allergies since my fibro dx that it's frustrating to figure out what to eat every single day. I get extremely hyper on coffee but never noticed pain level changes. It's strange that many pain killers have caffeine in them...Excederin, Anacin, Percogesic...supposedly because caffeine helps relieve pain.
I personally have not noticed any diet change worsens my symptoms. I won't give up the chocolate!
I usually drink tea but I do have coffee occasionally. I think it's the caffeine in it that is supposed to affect some people. It doesn't affect me but it could affect you. There is caffeine in chocolate, too. Have you noticed more pain after eating chocolate? If you do, you could be sensitive to the caffeine. Try decaffeinated coffee and see what happens. You will at least get the flavor you love.