I have my own theories about spall versus blade-derived gunflints not directly matching with those of the venerable and appreciated Hamilton. Things to consider: At sites dating from 1760 through 1820 it is not uncommon to find both spall type and blade type musket flints together in various ratios. This goes against the idea that one type is early, and another late, though it is true that spall flints disappear for sporting guns in the late flint period.
The proposal that French knew how to make blades and the English did not: Contrary to this hypothesis, there are plenty of French spall flints, and plenty of English blade flints mixed in with English spall flints. I have never bought the idea that the English did not know how to make blade flints; that is was a French trade secret. Since Cro-Magnon times, all cultures have known how to make flint blades.
I obtained a lot of original English and French flints from Nepal where they were found in a warehouse and spall and blade flints are both present in good numbers. These likely date to post 1800 based on the history of the region.
There is no doubt that from certain types of cores, using the blade technique is more efficient. But when a core is small or nearly exhausted, a spall flint can still be obtained after no more blades can be struck off.