Dark oil will often stay dark, even when well filtered, and de watered.
When the dark oil is well filtered and dewatered, even though it's still dark, you'll be able to see quite clearly through it, if it's in a jam jar. Sort of like looking at an old sepia photo.
It'll have a crystalline like appearance about it, where as the untreated oil just looks dull and dead.
Out of really dark, but otherwise quite clean oil, after adding the petrol and settling, you should get quite a noticeable sludge at the bottom of the container.