A black T-shirt has issued a statement about being a black T-shirt. The available evidence appears to support it. We can probably put the investigation on hold.
This T-Shirt Identifies As A Black T-shirt takes a very simple observation and gives it far more ceremony than it needs. You can already see the object. Then the object uses words to describe itself, with the confidence of something contributing important new information.
The extra step is the joke. A plain label would be practical enough on a shelf or in a stock list. Put the description on the thing itself and it becomes oddly formal. The shirt seems determined to make sure there has been no misunderstanding about the shirt.
The object gets a speaking part
Most messages on clothing point away from the clothing. A band name points toward music. A place name brings somewhere else into the picture. A slogan offers an opinion for the wearer. Here, the sentence turns around and points at the surface it is printed on.
That makes the garment part of the wording. The phrase needs the T-shirt underneath it to complete the thought. Read the same words on a loose piece of paper and the relationship changes. On the shirt, the subject and its description occupy the same space.
It is a small bit of absurdity, but a satisfying one. Something usually treated as a quiet object has become extremely clear about its own classification. The seriousness of the sentence makes the obviousness more noticeable. A shorter label would probably be more efficient and considerably less amusing.
One unnecessary clarification
The design has room for the reader to enjoy the wording without solving it. You can spend a moment thinking about labels, language and the things we make objects say. You can also simply appreciate a shirt taking itself far too seriously.
That self-reference links it with Your T-Shirt Sucks. There, clothing appoints itself a critic of other clothing and promptly puts its own opinion in the firing line. Here, the shirt keeps the discussion closer to home. It has chosen itself as the subject.
Do Not Read This gives the reader a different little trap: the instruction has to be read before it can be followed. Both designs get their humor from the act of reading something on a shirt, rather than attaching a separate joke to an otherwise unrelated object.
There is a useful modesty to a joke this circular. It can occupy a few seconds, produce a smile and leave the rest of the day alone. The T-shirt has made its announcement. The T-shirt is still a T-shirt. At least the paperwork is unusually visible.