If you want to have an even patina over the whole blade you´d have to dip in the solution completely with not too strong a solution and a clean blade.
I did it with vinegar once and with citric acid.
It really depends on the acid content of the solution and how dark you want it.
I used a vinegar concentrate with water and left it in for 2-3h. You can of course take it out, have a look and "throw" it in the mix again.
When finished, get off the acid with water.
In the picture you see its colour and the extra patina towards the edge developed by use.