Well before you can enforce flag rules the entire comunity needs to agree on what each flag means and what the approate reaction to it is or this needs to be defined by the devs.
The blue flag is one of those that really has a lot of different meanings to different people. To some it means get out of the lapping drivers way, to other it means hey there is a lapping driver approaching, drive a predictable line and give them room. I do not think you are going to get everyone to agree as the comunity is split down the middle. Because of this defining a "Black" flag is going to be diffacult if it pretains to blue flags.
My personal take on this is that the blue flag is nothing more then an indication to a driver that lapping traffic is approaching, drive in a predicible manner and leave room. With that definition the lapping driver then is responcible to safely passing the laped car when safe to do so. Now under a system like this I would define the rule that if the laped driver blocks or un-nessasarily delays the lapping driver then they should recieve a black flag resulting on a drive through penalty.
The problem then become how does a program detect the intentional blocking or delay of a lapping driver? I do not think it can. I have raced for several laps flirting with a blue flag and not had a lapping driver get close enough to effect a pass. In this case I am not delaying the driver nor blocking him. Another thing to consider is that taffic and slower drivers are a very real part of racing. They can and do become part of a smart racers strategy.
A smart racer can use traffic to effectly block another driver they are racing for position with, at the same time if a you are racing for position and a lapping driver approaches and you slide over to allow them past its very easy for the driver you are in a position battle with the slip in behind the lapping driver and pass you. In which case allowing the lapping driver to pass cost you a position battle. This happened alot to me in F12k2, the AI would even use this technique and it sucks hard.
The point is that very carful though needs to be put in to this. The people making the decision need to look at alot of different perspectives and how each rule can be exploited or how it may effect every one in every situation. If done without enough care and though it may cause more problems then is solves.