It's not an item, it's an NEI option. It's called Highlighted Block Tooltips, click it to set it as true to see it. I must note however that it has a minor derp when it comes to Greatwood Leaves and Shimmerleaf flowers, so you might want to turn it off when approaching those (I don't think the effects are actually serious, it's just a minor graphical hiccup).
Hope I helped.