It sounds like tar deposits.
I would be worried that the tar has migrated to the outside of the chimney.
This is TAR and is usually deposited from the products of combustion on the INSIDE of the flue.
Your observation suggests that the flue is broken or missing and the chimney may be smoking IN the attic.
Some background information -
There is another thread on AAM where a posted discussed the black tar residue in chimneys.
You'll have to search for it as I don't have it to hand - I posted to it I think.
http://www.stovesonline.co.uk/chimney-tar-soot.html
[broken link removed]
http://en.wikipedia.org/wiki/Creosote#Build-up_in_chimneys
Please note the general comments about reduction in efficiency.
Please note the comments about chimney fires in the last link.
No connection to any of the companies named.
ONQ
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.