Some TMNT stuff was always meant for grownups.
The Blind Ninja is an unnamed Foot Ninja first appearing in Tales of the TMNT volume 2 issue 5: Blind Faith. He is never seen in person, as his very first act is to blind Leonardo when their fight begins. In the comics, the blind ninja is only ever depicted in a series of stark black and white silhouettes to simulate Leonardo's sightlessness.
Before the events of Blind Faith, this ninja was among the various Foot Ninja that the turtles fought sometime in the past. Leonardo blinded this ninja in battle, though Leo himself can never remember when or where this happened, much less who this ninja was. When the ninja was blinded, his other senses compensated and became sharper, but the Foot Clan retired him from combat and put him to work polishing other ninja's weapons. This indignity motivated the blind ninja to seek revenge against the enemy who maimed him.
In Blind Faith, the blind ninja attacks Leonardo, and the very first thing he does is use drug-tipped tetsubishi to blind the turtle through his bloodstream. Leo is forced to fight his opponent completely blind, but soon realizes his opponent is just as blind as he is. Fighting on a city rooftop, the blind ninja tells the turtle his story. The battle changes course, and Leo manages to impale the blind ninja on his katana. But the turtle is horrified to realized that his victim is not the blind ninja, but a nearby innocent homeless man. This had been the blind ninja's revenge—to take the turtle's sight, and then to take the turtle's honor from him.
In Tales of Leonardo: Blind Sight, Leonardo is still blind weeks after the battle, and struggles to come to terms with the battle's consequences as he experiences a crisis of purpose. The blind ninja's words and actions continue to taunt the turtle. During a visit to the woods outside Northampton, Leo is revisited by the blind ninja, and is drawn into a fierce forest battle with him, but afterward cannot be certain this experience is not a dream or hallucination. Splinter suggests that his son's blindness may be more psychological than physical, and serves Leonardo his special brew of infini-tea to help him find the answers in his subconscious. In the end, Leo finally realizes he could not remember the homeless man breathing when he killed him—the man was already dead, having already been killed by the blind ninja and placed in front of Leonardo's blade to force the turtle to suffer the psychological trauma of taking an innocent life. With this revelation, Leo's crushing guilt is alleviated, and his sight returns on its own.