Cigar tar in clipped area.

Like someone alluded to already, I think it's not really "tar", but condensation from the smoke. Burning stuff produces water as a combustion by-product.

I don't know how/why this occurs sometimes, but I had this happen to me once and that black/brown liquid was THE most rancid, bitter foulness I had ever tasted. Ever since I've kept a piece of tissue paper on hand to dab the end of the cigar if I see it happening, and I try not to lick the end of the cigar or even wet it with my lips. I seem to recall getting a similar flavour just from the wet tobacco, but maybe I was actually licking some of this stuff off and not knowing it.