A couple of years ago I tested Blade Madrigal potency and found that it was +28 at 488 skill, which was the highest I could get at the time. Now I can get up to 521, so I decided to go test it again.
It turns out that Blade Madrigal goes up to +29 at 490 skill, then +30 at 508 skill. The next tier would be at 526 skill, which I can't hit yet, so I can't test it.
Since Sword Madrigal caps at +15 accuracy, it seems reasonable that Blade would cap at +30, but I'm not sure. Someone else want to test it for me? Kirschy?