Too dark.. overfed in some shape or form. N, Mg, others cause over-lushness too.
The curve in the leaf blade indicates a possible fluctuation in pH, too -- this will cause a varying growth rate so you get a warped leaf or two.
Damage along vein and not interveinal, so maybe some sort of root zone issue too.
It's important to remember that diagnosing leaf symptoms is not discrete. Symptoms often have more than 1 cause and you need to reference your fertilization methods, watering procedures and how it progressed to widdle it down to a more likely answer. Looking at 1 leaf is insufficient information.
Everyone needs a leaf symptom chart, unless you memorize it. Keep track of ratios of what you feed -- either ppms or weighted average of the percents off labels etc -- and recognize how it progresses.. immobile vs mobile can help eliminate possibilities, for example. Even if you still ask questions, initially, always try to reference and cross-reference all the data you have on your own too... it's the only way to learn.