With Xcode 10.2, I could select the "Authors" view, then select a previous git revision to blame from the bottom picker:
But with Xcode 11 beta, when I select the "Authors" view, I'm unable to select a past revision to blame because the picker is missing:
Was that feature removed, or are there new steps to do to blame the lines of code for a past revision? To illustrate it even more, I'd like to see in Xcode 11 a past Blame like GitHub does when you blame a specific previous revision.
for a git blame version click Editor->Author as it, sure it moved
you can select
the icon located in top-right of Xcode to find a previous revision
Select the 
icon in the window title bar to enter "Code Review" mode.
git log (History) is available at Cmd-Alt-2. There you can open a specific commit in code review mode via the context menu.
Mine wasn't showing up until I went to Editor > Show Last Change For Line. After I selected that, the 'Authors' row appeared in Editor > Authors, as well as in the 'Adjust Editor Options' icon in the top right of the page.
Answer to the original question:
You can't. Since Xcode 11, you can only git-blame your working copy. Note: This is my suggested answer after a lot of trying and googling. I'd be happy to learn the opposite.
Further information:
Many people, including me, seem to land on this page because they don't find the git-blame feature AT ALL in Xcode 11.
In Xcode < 10, the version editor came in three flavors: 1) Comparison view (=side-by-side diff), 2) Author (=git-blame) 3) Log. In all three flavors, you could choose a specific revision to look at.
In Xcode >= 11, the version editor (now called “Code Review”) only encompasses the comparison view (=side-by-side diff). The Code Review mode is toggled on/off with the double arrow button in the tool bar.
The Authors view (a.k.a. git-blame) is now a side-pane of the normal editor and can be toggled on/off in the Editor main menu. THIS OPTION IS NOT AVAILABLE WHEN YOU ARE IN CODE REVIEW MODE! For me, this caused confusion when looking for the Authors view.
The fact that the Authors view is not part of the version editor might be the reason that we cannot choose to look at arbitrary revisions. I have filed a suggestion with Apple to re-enable this feature. You should consider to do the same to give the issue more visibility.
In Xcode 11, the previous "Authors" menu item doesn't appear by default in the menu set. The simplest way to make it appear is to press the default key bind shift+control+command+A. After that, you'll find that "Minimap", "Authors", and "Code Coverage" now appear in the "Editors" menu.
If you go up to the top of Xcode, go to "View" and tap on "Show code review", you'll be able to get the git history with author information like on older versions of Xcode.
