I've never heard of a Slate Budgie before
Here's a picture of a slate budgie from google
And background info on the mutation:
"Slate is a color-adding factor similar to grey and violet. Slate produces a very dark bluish grey in white-based budgies. The darkness of the slate varies slightly according to the dark factor of the bird. Slate, like violet, can be present in a green (yellow-based) budgie, but only produces a darkening effect. True slate only appears on blue (white-based) budgies. This variety is extremely rare."
Every website I went through, says that Slate budgies are really rare
Does this man have 200 of them? I find that hard to believe.
Although if you live in the UK, this may be true. There has been efforts to try and revive slate budgies
Here's some info on it
Good luck trying to get one
Hope this helps