I'd create an entire image made purely with smudges. Like putting loads of charcoal/chalk on your fingers and doing it that way on a coloured piece of paper. Charcoal/chalks for tonal stuff, you know? Plus, they smudge better.
When I couldn't think of stuff to do for a theme, I think of what I want to draw, and then I try to think of ways to merge that with the theme. Not add smudges to the idea, like you said this is a last resort and usually looks bad, but actually merge the two ideas.
I'm fascinated by blood at this moment so with your theme I would paint smudged blood or something. A hand print, a hand, smudging blood on the wall.
A while back I was fascinated by strong light, I would have painted a person (a clear silhouette) standing on a city street and the whole city slowly turning into a fuzzy smudged mess.