To expand a little on bkdavy's comments, dark black exhaust often means there is metal-to-metal contact somewhere in or on the engine. I've had mufflers come slightly loose and was alerted to this because it caused the thick black goo to suddenly appear on the plane. I've also seen this when engines work themselves loose on metal engine mounts, but that's not as common in my experience. It's simple enough to double check your muffler (inside and out) and engine mounts if applicable. If that's not the problem, I'm sure someone else can give you more hints.