There are other things to consider than the flux. The black flux is
better on ferrous materials than the white but that will work too.
Don't ever rely on flux to clean the metal surface. It keeps the
surface oxide free when heated.
Always clean the surface mechanically before silver brazing. On a
forged steel item you may or probably want to retain the black oxide
surface from the forging. This is an iron oxide, magnetite that is
mill scale when thicker. then just spot polish the area that you want
to solder then flux.
The balling up of the solder on a clean surface indicates first that
you are melting the solder before the base material is hot enough. If
the surface is dirty that does it too.
The rules may call for a neutral flame, but a very slightly reducing
flame will work better.
This insures that the flame isn't oxidizing. With silver brazing you
want to heat the part so it will be at soldering temperature before
you apply the filler metal. On very heat conductive materials-- brass
and copper, silver etc. this requires a big brushy flame depending on
the object mass.
On a very poor conductor like stainless steel and a little less for
steel, you don't need to get the whole piece hot just the area you
want to join. In this case a much smaller tip is what you want as you
will get the joint zone hot quickly and then be able to run the
solder around the joint. Keep the heat toward the big side. If you
don't stick and the material gets black -- clean up and start over--
fooling with it won't help. Practice.