I have seen sand blasting grit for $10 for a 20lb bag what size grit would be suitable for HO. The store is reluctant to open a bags for me to check grit sizes.
Anything 1 mm across and larger is too large. Stuff smaller than 0.5 mm is too small. You would want bits averaging 1/32" - 3/64".
This is not to say that material close to 1/16" would not work…it surely could…but it would be oversized for typical ballast in scale.
Based on the tiny size I have seen for sandblasting grit in the past, I think it woudl be very suitable.
I think the problem you’ll have is getting a suitable color. Most sandblasting medium I’ve seen is too brown or too black.