Temporary brown?

Hey guys, so i got a question here! i have blonde hair (coloured) and as you all know halloween is coming up, me and my partner are both going as vampires to a party but blonde doesnt look very vampirey! I was wondering (may be a stupid idea!) if i got a brown wash in wash out colour, like the cheap ones you can pick up in superdrug or boots that last about 6 washes and put it on my hair what would happen? would it go a nice (temporary) brown? or would it be a strange colour of blonde? Also if it did go browny how would my hair be once it started washing out, would it stain my blonde hair or anything like that? i know i sound totally stupid but ive searched all over the net for answers and cant find any anywhere so i thought id seek advice from the people who know best!!! I really dont fancy wearing a wig and being all hot an stuff all night and worrying about it slipping off!!