Sorry, can't tell much from your picture, got any more? Bigger?
Soot is usually (if not always) caused by incomplete combustion.
Was there plenty of combustion and makeup air?
Did any other appliances or the return air of the furnace itself upset the apple cart with all the doors to the equipment closed?
Is the flue open and sized correctly?

__________________
Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002