True blue prints produced a negative image of the original with white lines on a blue background.

Blue line prints replaced classic blue prints probably 45 or more years ago. Blue line prints produce a positive image - blue lines with a white backgroung. The blue line prints utilize the diazo process. The paper is purchased precoated. While it is light sensitve, it rquires a very intense light source. As stated earlier, it uses ammonia as the developing agent. There is no fixing required and image is not very permanent.