My initial assumption was the Himalayas to be stored in the Sanctum there under mystic guard and key. It seems reasonable the Darkhold if it couldn't be destroyed, that it be constantly protected and watched by people unable to be tempted by it.
Since the visual effect used by Reyes resembled the magical sling-ring portals created by Dr. Strange and other mystics, I would imagine he was taking the book to the mountain stronghold of the former Ancient One.
Update: Even after checking around, no one has been able to define conclusively where the Darkhold ends up. A number of Wikis imply the Darkhold was taken back to the Dark Dimension, but that doesn't make sense since the MCU has made the Dark Dimension extremely hard to access during the Doctor Strange movie.
While the Ghost Rider appears to have significant supernatural power, it seems unlikely he would try and move the Darkhold to a place where Dormammu would welcome an opportunity to review its pages.
Its current location remains unknown.