.38 Short colt will fit in the chamber, but somewhat loose. maybe it will be centered enough to get a good primer strike, maybe not..... Since it's a blank, I suppose if it did get a good center hit on the primer, and went bang, shouldn't be any problem. Sure wouldn't recommend shooting .38 short Colt ammo with a bullet, though. The original load used a .375 bullet, while a .38S&W used a .360. Short Colt case is also .375 diameter, while S&W is about .010 bigger
As a side note, when I was a kid, my dad had(I still have it) a mostly unmarked .38S&W break top similar to yours, only had a serial number. Probably a Spanish copy I guess. He had a partial box of shells with it, and we shot some. Nowadays I know that some of them were proper S&W rounds, some were Short Colts. Short colts swelled the cases a bit, and spit lead at the cylinder gap, and wouldn't hit the broad side of a barn, but they didn't hurt the gun. But now that I know better I sure won't do it again.
But as I said above, I doubt that blanks would hurt anything, since bullet diameter wouldn't be a problem anymore