I used the original stuff from Herters, and I have used the new/old from Brownells.
Yes the original works great. Most all the older 'quick rust' solutions work good because of the mercury compound in them. As noted,,that's why the poison lable is on it.
I stopped using the original back when Herters was still in business as I was concerned with the amount of mercury contamination I was spreading about. I did alot of rust blue,,most all of it 'quick rust'.
When Brownells came out with the new/old Herters Belgian Blue, I thought (because of the times) it possibly was reformulated w/o the merc much like the BirchWood Casey quick brown had been. I gave it a try.
From what I can see, it still has mercury compound in it. A swipe of the soln onto a piece of warm polished brass or copper leaves the mercury plated firmly to the surface. (A problem you have to watch out for if you do use it or any with merc if you rust blue over any brass, copper, silver, gold, etc inlays. It does not come off easily,,not even with the carding).
It has the smell of the old stuff too when applied to metal that's too hot. A very definite smell.
So I gave up on Brownells Belgian Blue too. I have enough problems holding things steady w/o adding merc to the system and my surroundings. Maybe I have a bit too much already.
It is in the carding dust, it doesn't go away. So will be on the floor of your shop, your clothes, shoes, tools, bench, lungs etc. It gets tracked all over.
For quick rust blue (hot rust blue) I've used Mark Lee for the past 20 yrs or so with complete satisfaction.
I do alot of small parts with quick rust of late and slow rust the bbls.
He also makes a quick rust blowning soln but I've never used it.
It's available from Brownells too
For slow rust blue & brown I use Laurel Mtn and have done so for at least as long.
Just my thoughts on mercury compound blueing solutions.