Hard to tell from the photo, like you say!
Some thuringians can be very dark - if they have been used with oil (I have seen a lot of old ones that were) then they appear really dark.
Slate stones used with water usually appear greyish, but once again if they have been used with oil they appear quite dark. A lot were used with oil.
The very fine, often soft, man-mad hones are very dark when used with oil - some appear quite black. Oily deposits that have hardened can make it hard to identify the stone as man-made.
The seller must have some reason to call the stone a slate-stone. Historically, the term 'honing slate' has encompassed a wide range of stones, including thuringian, arkansas, charnley forest, etc, so the term might not be helpful as the very hard novaculites like charnley forest and arkansas are quite unlike the softer thuringians.
If it is actually slate it will be around 5k - 8k: most likely towards the lower end of the scale.
Regards,
Neil