I want to darken my walnut stained wood door and trim. Could I rub with dark paint and wipe to give it a rustic look? Or should I lightly sand and go over the existing stain with a black stain to give it the brown/black finish I’m looking for?
Yes, you can do this. I have used that method when trying to make new trim match older trim. Another trick is to use an oil (not polyurethane) satin varnish with a small amount of stain. Put in an amount of varnish you think will be correct for your project. Add the small amount of stain color of your choice and stir. Apply to the surface always following with the grain. Do not sand the existing finish as this will cause you to leave too much of this "varnish stain" and caused too much darkening.
Would be a good idea to use a non soapy degreaser to clean the trim first.