The Old Blood, the prequel
Wolfenstein: The Old Blood came out on Steam on 2015-05-04, developed by MachineGames, Metacritic 76, $19.99. It is a standalone prequel set before The New Order, not a sequel.
A year after The New Order, MachineGames shipped a second Wolfenstein game that is not a sequel. The Old Blood is a standalone prequel: it happens before the events of the first game, stars the same character, and runs to eight chapters.

The record
| Field | Value | Source |
|---|---|---|
| Steam appid | 350080 | Steam store API |
| Steam release | 2015-05-04 | Steam store API |
| Developer | MachineGames | Steam store API |
| Publisher | Bethesda Softworks | Steam store API |
| Metacritic | 76 | Steam store API |
| Price | $19.99 | Steam store API |
| Length in chapters | 8 | Steam store description |
| Platforms on Steam | Windows only | Steam store API |
The Steam description calls it a standalone prequel that spans eight chapters and carries the studio's hallmarks: action, story, first-person combat. It is a shorter, cheaper game than either of the numbered entries, and it was priced that way from day one.
What prequel means here
It sits earlier on the calendar of the fiction, not later. If you are playing the MachineGames games in release order, you go New Order, then Old Blood, then New Colossus, and the story jumps backwards in time between the first two.
| Game | Release order | Story order |
|---|---|---|
| Wolfenstein: The Old Blood | 2nd | 1st, set before The New Order |
| Wolfenstein: The New Order | 1st | 2nd |
| Wolfenstein II: The New Colossus | 3rd | 3rd |
That structure is worth keeping in mind when people describe the series as a trilogy. The trilogy is three games of story, delivered across four releases if you count this one separately, or five if you include the 2019 pair.
Where it stands
The lowest-rated of the three campaign games at 76, and the shortest. It is also the cheapest way to see what MachineGames' Wolfenstein feels like, which makes it the usual recommendation for someone who has never played one.

Nothing about it has been referenced in the Wolfenstein 3 announcement. The script page names B.J., Fergus and a new character called Otto, and mentions no location from this game.