Color developers sometimes form a black or brown tar due to aerial oxidation of some of the organic components they contain. A brief soak in dilute acetic acid (stop bath will do) will get rid of the tar completely.

I have not seen this myself since the days of CD-2. CD-3 and CD-4 are so much more soluble in water, as are their oxidation products, that tar formation is not a problem. Unless, could it be.....??? Have you been talking to Kodachrome processing people? Yeah, Kodachrome can form colored gunky tars on the equipment. That must be the source of this.

However, remember in all processing, Color and B&W, that the final wetting agent can form a jelly like material on reels if the reels are not washed properly in hot water after use.