Dark, heavy (oily) ashes are usually from two things.
First, most likely the soil wasn't properly flushed of nutes before harvest.
Second, likely the bud wasn't properly cured.

Some folks do not cure their indica's, thinking they smell and taste fine. For some strains, that's true. Well...except for the uneven burning, dark-oily ashes that are more embers than ashes, and that ammonia taste that ruins the flavors and give you a nice headache. (possibly why you think they may have been sprayed with something...?)

If properly flushed and cured there should be a light and loose greyish to whitish ash left in the ash tray. Joints burn more even, Flavors are sweeter, and no chemical aftertaste. :thumbsup:

There's a curing thread here somewhere. You can moisten your buds just a tad, put in a glass jar, and finish curing them. (if you haven't already smoked it)