League of Legends (LoL) Question: Question Concerning Masteries

I personally never take Block on junglers because it's a lot stronger in lane. Block only pays off if you end up taking a dangerous amount of damage from an enemy champion through autoattacks, which for junglers often isn't the case. It's pretty good in 1v1s, 2v2s and 3v3s though if the other jungler is also present during a gank.

I'd say it's mostly preference but I don't think it's necessary. There are other good defensive masteries you can get with 21 points.

block and unyielding isn't good on a jungle champ, you usually have it for top laner and sp, where the amount of AA harass is a lot. Personal pref aside, it's just not logical to have it.

I would take 21/9/0 on Jarvan for early gank damage. But if you're a tanky jungler, yes, take those masteries.