It features a salt glazed gray finish on its exterior with cobalt blue blueberries in a basket on its front surface.
The interior of this mug is finished in a very unique green to brown gradient.
I could not find any markings or stamps on this mug to identify the manufacturer, however I strongly suspect that it was manufactured by Salmon Falls Stoneware.
Condition note: this mug has no cracks, scratches or crazing. There is a super tiny nick on the rim. Please see the red arrow pointing to the location of this in the 5th photo, though it will be very difficult if not impossible to see in the photo.