If you’re using an IPS LCD monitor you’ll probaby enjoy the nice different shades of gray, but in a non IPS LCD (like my LG Flatron L246WH) the first block is barely visible and the one on the second article is completely invisible unless I move my head up/down a bit.
Please select less problematic colors.
On a related note, in my case I would simply prefer to have the option to not show that block at all and use the space for the content.
(Or maybe you could narrow it and show just the icons, with the text showing just when you “hover” the mouse). 