This isn't one of the traditional blues tunings, but it's one of my favorites. I hardly invented it, of course, but started using it without having previously known of an example of somebody using it. I "discovered" it a couple of years ago when I wanted to play & sing "I Wonder As I Wander" and thought G minor a good key to do it in. I just flatted the third (second string) of Open G, so from low to high it's DGDGBbD. (Bb reads as A# on most tuners.) "I Wonder As I Wander" was easy to pick out and play, and sounded good. Messing around a bit in this "new" tuning I quickly ran across a couple of other tunes that just practically played themselves: "Summertime" and "Carol of the Bells." Excitedly I went to my next guitar lesson with it still tuned to G minor and played "I Wonder As I Wander." My teacher didn't seem particularly familiar with or excited by this old Appalachian folk Christmas song, but then I said "Check this out" and started playing a basic version of "Summertime." He watched me through one verse of it, and said "Give me your guitar!" Never having played a guitar in this tuning before, he proceeded to just wear it out with cool improvisation on this old standard. "Wait, there's more!" I said. Played "Carol of the Bells," which also got him pretty excited. It works well with or without slide, I finger most of it anyway and just use the slide a little.
We messed around and found other things that work great in this tuning, including the old Merle Travis song "Sixteen Tons" made famous by "Tennessee" Ernie Ford, "Ghost Riders In The Sky," and a slightly bizarre invention of mine, The Drifters' "On Broadway" done in a minor key. (I call it "Off Broadway," 'cause it's just a little "off.") Played as a slow slide blues, in minor key, that really sounds like the blues!
I use this tuning to play "Let All Mortal Flesh Keep Silence," and suspect that I could just be the first person to ever play that one on slide guitar.
"Ain't No Sunshine When She's Gone" works well in it, as does "The Thrill Is Gone."
My latest "discovery" came late last week when I was sitting in the back office jamming on "Summertime" on my tricone resonator. A coworker who grew up in New Orleans came running and asked if I was playing "St. James Infirmary." I told her it was "Summertime" (I was getting fairly jazzy and the tune was sort of buried), but I hunted around on the Web and found an MP3 of "St. James Infirmary" as played by Roger McGuinn. Thought it did sound kind of like what I was doing. I listened enough to get the basic melody in my head and went back to messing around on the guitar. It's stuck in my head, and I wake up hearing variations on it. (Too bad I can't remember them when I pick up my guitar!) But it's coming along, and I'm having fun with it. I like this song. (Which according to some notes on the Web was first published in 1912 and is arguably the first published blues song.)
This tuning works well as Open A minor on lighter-strung guitars. Same thing a step higher. EAEACE.
It's fun to try a new tuning. It'll give you new ideas and you'll run across easy ways to play things you might not have been able to do in another tuning. Cool stuff!