O.k I don't want to cause no shit storm, but I think most growers agree these days flushing the plants of nutrients before harvest to create a cleaner smoke or whiter ash is all a myth or bro science at best.
Everything marijuana uptakes the entire grow cycle nutrient wise stays inside that plant. There is no way to..."flush" the nutrients from the plants, you can only over or under feed.
Most peoples flushing techniques in my opinion are just starving the plant at the end of its life cycle, causing it to use up remaining nutes and start to feed on itself. Which isn't necessarily a bad thing... I usually let mine go yellow or whatever color that pheno goes when it gets to 8-9 weeks flower. Still lightly feeding till the end.
So I never flush my plants...And my flowers usually always burn clean and white ashed. I think its more about improper drying and curing techniques that has your smoke not staying lit and burning black.
Drying and curing is an art like growing is, it takes lots of trial and error in most cases to get it just right. You must have got a bad batch. Shoot at least it still gets you stoned...try mixing it with some better burning flower?