SAAMI ( Sporting Arms and Ammunition Manufacturers' Institute) set pressure limits (along with a lot of other standards on ammunition and other firearm related specs). Following the SAAMI spec is voluntary but pretty much all firearms manufactures and ammo manufactures that are selling in the US follow these specifications.
38 Special 17,000 psi (Transducer)
9mm 35,000 psi (Transducer)
357 Mag 35,000 psi (Transducer)
You particular Blackhawk can easily take 38 Special ammunition loaded to 35,000 psi but there are 38 Special revolvers out there, that pressure that high would destroy. Since they were designed for 38 Special ammunition as specified by SAAMI. Your gun was designed to take 35,000 psi and chamber for 357 Mag and it just happens that the shorter lower pressure 38 Special will safely function in it. If you want to reload you own "super-hot" 38 Special you can but please understand the risks you create if your ammunition ever falls outside of your control it could cause issues for someone else that does not realize that ammo is loaded over SAAMI spec.