The pink eye dilute gene (which gives champagne) turns a rat into a much paler version of itself. If you have a black rat, then add PED, you get a diluted black. Black is a cold colour, you add PED, you get a paler but cold colour. Chocolate is a warm colour, and a bit paler than black. If you add PED to chocolate, you get a paler and warmer colour than a version based on black. It’s not that black is making it darker – it’s that the chocolate gene is making it paler.
I’ve seen people advise adding black to a Siamese line to darken up the points, but it doesn’t make any sense and it doesn’t work. Adding black can be useful because you can see that blacks have dark feet, so you can use it to help to eliminate white feet from the line. But it won’t make the points any darker, it will just help you to breed white toes out of it.
The depth of colour of the Siamese shading is dependent on hundreds of other little modifier genes. You want to breed in as many of those as possible to your Siamese line to get good dark points. If you have a black outcross from a dark Siamese line he might carry lots of these modifiers so be useful for darkening the Siamese, but his Siamese brother would be just as useful