Preliminary Review -
OK, I ordered 5 PR4 Blogs to test them out. Honestly, I'm not sure how I feel about it at the moment, but more on that later..
Communication was good and turnaround time was great. I placed the order, the next day I was contacted for my Tumblr email so that I could receive the blog invites and start the push to my account. I am now a member of several Tumblr blogs that are indeed the advertised Page Rank. I am awaiting promotion to admin, then OP should leave the group. Everything is as advertised, so far.
But...
The OP explained that these blogs have no posts in the thread.. Initially I thought, that's not possible with NO content but then he posted an example and everything looks legit. I contacted the OP and asked his opinion on whether these will stand up after the expected May PR update. His answer: "I had a load before the previous PR update and all kept there PageRank just fine

"
I went ahead and bit on it. I ordered 5 PR4 blogs. I received 4 @ PR4 and 1 @PR5 and bonus PR2s. So the OP over delivered on the PR5.
So, I started investigating inbound links to the PR5 and I found a ton of links pointing to old posts that no longer exist. Hmmm, odd... So I check out the Way Back Machine at archiveDOTorg. Come to find out that this blog used to have a massive following on Tumbler with a ton of likes/follows/notes from Tumblr users in the past. Now it's all gone? So what happened...
To me, it's pretty obvious what is going on. Google search site:*.tumblr.com, find blogs with PR. Keep looking (or use software) to find high PR blogs that have had the Tumlr URL reclaimed in some manner (Blogs that no longer exist. Either closed for inactivity or reclaimed for a number of reasons.) Then create a blog with that same name, claim the Tumblr URL, then sell them.
What you end up with is a URL that matches the high PR blog that USED to exist, but NO WAY to recreate the old blog. Even if the blog is well archived at archiveDOTorg, Tumblr uses post numbers before the post title in the URL, so you CAN'T recreate the post on the same exact URL even if you know all the post titles , URLs, and contents.
Will these survive the next PR update? I can't say, but I know that when you buy a high PR domain from someone else and remove all content that was linked to, you generally lose out in the end. I know for a fact that most PR to individual blogs on Tumblr comes from the interlinking of the comment, notes, and likes from other Tumblr users. All of these no longer exist.
Know what you are buying. If you are OK with it, jump on in.
The OP gave me exactly what was stated, and the blog I was researching does indeed have a few external links pointing to the proper subdomain MYBLOGNAME.tumblr.com. I just wish I had known a little more info about how there were attained before blowing most of my budget for this month. Will they survive the update? I guess I'll find out.
I do invite pokerJK to chime in. How many Tumblr blogs did he try this with before the last PR update? How many survived the update? Did he do anything with content to salvage the PR, or did he just leave them empty and all was good?
I don't mean for this to sound malicious in any way. I just want you guys to know what I have found in my research after purchase.