Black, blue, chocolate and lilac foxes are tans with the chinchilla gene, as PPVallhunds has said, but 'at/* cch/ch' would be a burmese fox (basically a brown fox with dark points on the nose and tailset). Black, blue, chocolate and lilac are the only 'true' foxes because these are the only varieties in which the change of colour is restricted to the belly. A dove or champagne fox actually looks like a pink eyed cream fox (very pretty though, I must say) because the chinchilla gene bleaches dove/champagne pigment.
You can make agouti based foxes with the Aw gene (white-bellied agouti), so an agouti fox would be Aw/* C/*. As soon as you add something on the c-locus (Siamese, chinchilla, cream etc) you'll dilute the agouti top colour.
C-locus colours will always dilute a tan belly to white, so a Siamese tan (at/* ch/ch) would look like a Siamese fox.
Where you're talking about black spots, do you mean on the throat under the jaw? If so this is common in tans and foxes (and is a fault on the show bench).
To test breed, you're probably best off breeding your black fox to a black tan (that is definitely a tan), which will produce black tans. Then breed one of those back to the fox, which should result in a litter of which half of the babies are black tan and half are black fox. Test breeding is dodgy in your part of the world though because you have little access to true-breeding strains that don't carry anything, so be aware anything you use to test breed could be carrying genes that will skew your results.