they do challenge you for some races (beat their time on a tollbooth etc. and in the end you have to race them 1 on 1 if you win that you progress 1 place in the ranking and are able to get a other car (you still have to buy it!)
but this doesn't mean you only have to race 1 or 3 races for each person, in between you still have to increase your bounty, I think each blacklist caracter has its own bounty and as soon as you come close to that amount he will challenge you.
for those who think this is a spoiler, it is all info EA gave away in the past months, but I combined it to 1 piece to make in logical.












