Here is the short answer on the use of iron blacking on the grain. There are two main problems first is not enough tannin present and the second is casehardening the grain.
This quote is from Lamb’s book on Staining, dyeing and finishing.
“In staining blacks it is very necessary that plenty of the logwood infusion should be applied to the leather, especially if this is at all lightly tanned. Unless there is plenty of tanning and coloring matter to unite with the iron, the iron will combine with what there is of tannin matter in the leather, and render it brittle and liable to crack If too much iron is used, the leather may be completely ruined.”
Logwood is the answer when using this type of dye. Logwood is the best tannin to bind with the iron and keep it from changing forms and starting acid rot (or Red rot). If any of the iron is not bound to a tannin source either in the leather or in the logwood this rot is possible.
The second problem is very common when you apply enough tannin to stop the rot then it is easy to get some much in the grain surface itself that it can become brittle and crack. A very carful oiling is about the only way to stop this problem but it needs to be a mixture of heavy fats and not just a light oil.
One way to test yourself after dyeing is to remove all but the grain surface and soak it in a nutral water over night and check the PH. It needs to be no lower than 2.5 PH, a 3 or higher is even better. This is on vegetable tanned leather and it is naturally 3.5 to 5.5 PH on its own.
I agree with Al that blackening the flesh was easier and a safer way.
David Jarnagin email@example.com