I did try the blue flag app but didnt really like it, seemed to cause performance issues and for me having the full track map does the same thing and better.
With that I can see people coming up behind and if someone were to be lapping me it would show them a different colour to me I think. I dont think I have been lapped in AC yet so not sure exactly what colour it will be. But when I am approaching cars a lap down their arrow becomes brown and they will see cars lapping a different colour.
The track map is also great for when leaving pits in qualy to make sure you dont spoil someones hotlap, also lets you find some clear track. I did read also in the map thread the guy wants to have different colours for people on out laps in qualy as only they need to give way to you others on a hotlap can go as they want.