Probably the best blues song ever.Its in the scale of C minor & I've improvised a lot. Have a look. https://www.youtube.com/watch?v=Q1eIlJ3Hmv4
Can't access video right now, but this needs to checked out! Since I've been loving you has been my love at first listen song by Led Zep.
Issues with the video or youtube? Cuz I've been having trouble with youtube myself for the past couple of days. But looks like its working fine now.
You can listen to just the solo part here : https://soundcloud.com/sri-psychedelist/since-ive-been-loving-you-solo