The top is made from Bluestone, that is correct.

The product image shown above does not appear as only "grey" from here. This could be the device you're viewing the product on. There is a slight glare along the top, which could be what you're seeing. I've attached an additional photo below for your review.

The color is not blue or blue gray, but a dark charcoal. Is that correct? Thanks

It would be more of a dark charcoal mix.

If you do a Google image search for Bluestone, you will see that it's a mixture of dark grey, a very dark blue, and some black.