I just checked my 401a and my black top hat disks. The combination of B and O makes a stitch kind of looks like a blanket stitch. I have 20 of the black top hat cams, which is most of them. Of the ones I have, the #8 looks most like a blanket stitch. It is identical to the B and O combination on the built in cam stack. You have to really shorten down the stitch length to get something like you want. Neither one is as perfect a blanket stitch as you will find on a new Bernina. It is my opinion that this is the best you are going to get. Of course, I've been wrong once or twice.
According to the manual, the 401a came with the 1, 2, 3, 4 and 5 disks. None of those is a blanket stitch.
Good luck with your endeavor.