What is blue baby?

"Blue Baby' signifies a cyanotic conditon meaning a blue tinge to the skin caused by low oxygen in the blood stream.

Tick

It's often associated with nitrite levels. High levels of nitrite interfere with the ability of blood to carry oxygen. The reuslting condition is known as methemoglobinemia. It's characterized by shortness of breath and blueness of skin.

For more information, you can use Google :)

Or wikipedia which has a lot of links for further reading: Blue baby syndrome - Wikipedia, the free encyclopedia

As Unknown said, there are several causes, including cardiac problems. I do note that it is advised that nitrate (which I am assuming was meant) levels be checked when well water is involved.