M5Pots

With a commercial glaze it's complicated because you don't know the formula of the gaze. So you can either fire it hotter, or add a little frit  to it. 1-2% frit 3134 may be enough to get it to bleed, but you'd have to estimate the amount of glaze material if you're working with a premixed wet glaze and do a lot of testing. By adding frit you'd be voiding any food safety labeling, so you'd need to test it for durability and safety. That little bit of frit shouldn't make much of a difference, though. To make it less fluid you could add equal parts EPK and silica.
Zinc free is always a good choice when working with colors.

@M5Pots Thicken the underglaze strategically along with a fluid clear gets some control in the bleeding areas. Have also achieved interesting similar looks by using underglaze over bisque, and misting with a very fine mist to get movement pre glaze application. A judicial spritz or two from a well controlled air gun also helps move things in custom ways while spraying past stencils also allows for some custom feathering especially with an airbrush.

YEs I do like her things!
From what I have been gleaning in my search is that it is the glaze that will make the underglazes run. And that the Amaco Velvets will run with the correct glaze.
Sean O'Connell also fires at cone 6 and uses Jeff Campana's glaze receipe - available at Jeff's website - also check out Sean's things -beautiful also.
My problem is that I am limited to firing at cone 10 - so I am testing and asking for a runny-ish clear glaze for cone 10. 
So if you are firing at cone 6  - try Jeff's glaze - it should work for you - 
Also it really is just the blues that run - they have some cobalt in them - which is why she also adds blue to her blacks.
Thankyou so much for your input!
Jen

I took a class from Dawn Candy. She uses Amaco velvet underglaze & plainsmen underglaze. She said it was a runny clear glaze that gives the bleeding effect when that clear glaze is over underglaze with blue in it.  ( If I’m remembering correctly) She mixes blue in with the black underglaze. So far, it hasn’t worked for me. I think she said to add water to the underglaze which is trailed onto her green ware. I didn’t do that part. Maybe that’s why my hasn’t bled at all. She fires to cone 6, electric kiln. I have a runny clear recipe from her that she shared with us. I’m sure she wouldn’t mind me sharing it, if anyone is interested. Dawn also uses another super runny glaze that she wouldn’t share because it can really mess up the shelves!