The gas flow is not the problem when welding with 5356. The black soot that you are getting is called SMUT. This is more prevalent when mig welding with the 5xxx series filler alloys. What is happening is the result of metal vapor being produced by the intense heat of the arc, and then condensing on the base metal surfaces. the smut is a deposit of finely divided metal oxides. Although not harmful to the weld, it is a bit ugly. As you mentioned, you can wipe it off with a rag quite easily, but if left on the weld for a couple of hours, it can be tough to remove. Switching to 4043 will lessen the chance of smut formation, because it is mostly vaporized aluminum and magnesium, but there is still the chance of formation on and around the weld and heat affected zone.