Hey, I think I can help you! I started growing mine about 1 1/2 weeks ago. I encountered the same exact problem! I am also using eight 26 watt 6500k CFLs (but im not using 2700K) This is how I resolved it:: Stop Watering. lol easy as that. I let my soil dry out and my leaves perked right up in just a few days. The dark color appears normal and should be nothing to worry about. Its more than likely an illusion due to the outer part turning yellow. Speaking of that, (this may not be a problem for you) I had a problem with my leaves being a bit pale, yellow color. What I did was feed it my diluted urine - 10 parts water to 1 part urine. I know that sounds disgusting but the leaves turned green in a frame of 2-3 days (urea is high in nitrogen. It saves money too)!! So my seedlings are doing great!! One more thing: The size of your stem IMO looks a little stretched compared to your leaves. Make sure your lights are as close as possible without burning and you have a fan blowing. A good breeze strengthens the stem if that wasn't already obvious lol Hope that helps!! good luck!!!