oh dear, Funnily enough I did something similar this week!
I used to dye my hair black also - but the past few years I opted for a dark brown instead but got so fed up of it washing out too quickly and turning a redish medium brown - that I opted for Black!
It was a shock at first as it has been 3 years since I had black hair! But, it's grown on me - and it's not that bad after all!
But as for yours - I find that my colour seems to fade more quickly when I use hairspray!!
Also, apparently - "Hot oil treatments and dandruff shampoos are known to fade color." though I've not tried it myself!
Hope that helps a bit! - it shouldn't take long to fade

xx