I'm also an ibs sufferer..for about 25 years now. I've always drank coffee black and have never noticed that it produced a change in color of my stool.
Since it's only been a day or two, I'd wait and see if this continues..and definitely abstain from the black coffee for a few days while waiting. Black stools can be a sign of bleeding in the upper GI tract..so it's definitely something to keep and eye on and report to your Dr if it continues. On the off chance...have you taken pepto bismal within the last few days? Taken any OTC drugs or supplements? Pepto is well known to cause black stools..and some medications/food can cause temporary blackening also.
Let us know how you're doing after abstaining from the java.