I like Bernina, but I don't think any of the vintage machines has the blanket stitch; not until the 930 eighties model (I think it has it, but I can't confirm it atm). 1980s is sort of border line vintage, maybe if you are 16 years old it feel like that lol. I would expect any QE version to have this sitich, most current models in general?
The Elnagraph has a blanket stitch disk, I think it's No. 146 (I'm on the hunt for it). The Supermaitc is in my mind true vintage, and I suspect this is the only 1950s machine with back stitch patterns. I don't even know of a 1960s model who has it, though I suspect there are the odd exception, the Japanese jumped for the cam technology around that time. I have next to no knowledge of vitage Japanese brands unfortunately, though I have noticed a few nice machines and interesting features a few times.
If I rememeber correctly, you have some traumatic memories of a Supermatic giving you truble at some point. I can let you know, mine is working fairly well, maybe not fine tuned to your standards, but I managed to get the metal pulley with O-rings on!!! Still a bit on the noisy side, but compared to industrials it's really not (pedaling for some arguments in it's advatage). I'm keeping this machine for the time being, I think I might grow to like it. I just can't see me doing any free motion on it yet. If a later model happens to land on you, I think they were driven by two belts and no pulley.
If you are open to different techniques there is
a blog devoted to vintage macines and very true to the techniques at the time. It's a bit cumbersom to maneuver to the correct pages, but the result is quite nice. It feels a bit advanced too me, I have read it several times but I don't quite get it. It looks like it's all done by hand, but from what I read
she only talks about free motion and how to get the blanket stitch done on machine? She often uses a treadle model 15 and a 66 if I remember correctly.