Translation:I wear a blue shirt.
Because 青 means blue in this context (I guess it means blue in Japanese in most cases).
This word is the most confusing character expressing a color. In Chinese it can mean blue, green and black and usually confuses me a lot, so I avoid using it (in Chinese) except where the context limits its meaning sufficiently.
See sweg's comment. Basically "ao" doesnt cleanly match up with blue and includes some greens. In general its safe to use "midori" whenever you mean specifically green but there are two major exceptions -- both green traffic lights and vegetables are described as aoi, not midori.
More specifically, 蓝(lán) is blue, 緑(lǜ) is green, and 青(qīng) could be blue, green or black, depending on the object described and context. 青 is the more historic/literary word from back before Indo-European influences, when the line between blue and green was less concrete.