I think PvP in this game is silly.
But I also think drawing conclusions from a data set that contains only three points is silly.
Let me help you out.
Edit: Though, to be fair, I think the metric of achievements to determine someone's capabilities is pretty sketchy, you chose to attack him on arena cred though.
Achievements are a good means of determining what someone has done (or achieved...) but don't really speak to how they did it, nor what they care to do in the game. It qualifies you, but not having the achievements doesn't disqualify you.
Well Through achievements you can determine someone's overall experience and game knowledge, well unless they got carried. But generally i think people could get to 1800 without really trying. Then the grind to 2k then up to 2.2k starts. But really i only check to see if they atleast have 1800 rating experience. Then i could say oh this person have some form of experience with pvp he may know what he is talking about when it comes to pvp.
It'll be like "if" i said ret paladins are not very good in PVE while me not having any PVE experience, what i say about that class in PVE is not very credible.
From my experience i say blood DKs are not very good because well for i certainly never lost to a blood DK team in 2s, 3s, or 5s.
Some scenarios Blood DK / DPS : ok just go on the other dps because blood dks have weak peels and weak damage your team would just win. And for killing the blood DK it should be no problem because well no decent player will die to a blood dk hitting you for wet noodle damage.
vs Blood DK / Blood DK: well you can't die they don't have the dps to kill you, and well they may not die either. it's just a waste of time the game drags on and no body goes anywhere. Unless your team have the dps to kill 2 blood DKs but regardless of what comp you play they won't be able to kill you.
Blood DKs in 3s: the team with the blood dk will not get the kill on the other team because you can't create enough pressure or burst.
Blood DKs in RBGs : well it just gets out shined by prot warriors, but some could manage to get to high rating in RBGs playing FC blood.
Now DKs that main blood for pvp and not the other two specs usually don't realize this weakness which is why they tend to be well, poo poo at this game. There is an exception for Blood DKs that just play blood to troll. But outside of RBGs playing blood seriously is just, not effective.