I personally think blue is the hardest with going up and down those wallrides and keeping the car from sliding and black is comparatively easy, probably even the easiest part.

Good question. As green already said, the blue part is probably the hardest, followed by the white and red. Black and green have both a difficult starting jump and are comparatively "easy" after that. As to how long it took me to get a run without respawns... well.. lets just say it took a while :D