Can someone explain the blue?
Not sure if you mean the blue in general or the latest blue text but anyway:

Kinzo Ushiromiya is already dead. Therefore, the true number of people on the island is 17.
If we add an unknown person X in there, it becomes 18. If we suppose this person exists, the crimes will still be possible even if all 17 people have alibis!

Beatrice said earlier that there are no more than 18 persons on the island, which forced Battler to find the culprit among those 18(the family + nanjo + servants is 18 people). If Kinzo is dead, then there are only 17 people that Battler knows of, on the island, meaning he can reason that there is another person doing the murders, making them much easier to explain.. Most arguments thus far has been about alibis.

He also won't have to blame someone he knows, but he has been ready to do that for some time already either way now.

Edit: It should be mentioned that the blue truth isn't really "truth", it just forces Beatrice to deny it. That was probably obvious but I thought I should mention it just in case.