Re: Septic mainline broken...
My guess is that the blue color is to indicate what the line is to anyone digging in the area, so is it imperative that you use blue pipe, probably not, is it the best idea, yeah, it is.
Can't tell you why someone would stub out of the house with one thing, use two feet of another product and finally a third product for the run to the tank. IMHO, the house stub out should have transitioned directly into the blue pipe.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!