Open D is good for slide blues on a steel body instrument. It really rings. It's called "vastopol" tuning by some older guys here in Mississippi although they really don't know where the name came from. Someone said that in another post to this thread, about the old song "sebastopol". You can work a steady bass rythm with thumb and slide the upper strings at the same time if you're good, kinda like Booker White did in 1940 on "Bukka's Jitterbug Swing". It's a really fascinating style of playing. Best to use a guitar dedicated to open D and use heavier strings cause they have less tension on them.