The Blue Tops are specifically intended for marine applications, I believe they offer a starting and deep cycle within the Blue Top family. The Red Top is a starting battery and the Yellow Top is a deep cycle, although I've failed to notice any appreciable difference between the two in actual application.
The Blue top starting battery is the 34M and the deep cycle is the D34M. The starting battery has a higher rated cranking power and the deep cycle has a longer rated reserve capacity. Supposedly they are manufactured differently as to internals.