If it was safe, the data would be in the manuals.
Not necessarily.
There are, literally huge amouts of combinations that make safe loads, but space (and time and MONEY) for the manual are limited.
You almost never see light loads for heavy cartridges in the manuals, because those aren't what most people are looking for.
I've got some old loading manuals (Lyman, from the 50s) that have special "gallery loads" sections for some rifle cartridges. I don't see those in manuals from the more recent decades very often.
Never considered using Blue Dot in the .45-70, but I have used Unique. For "gallery loads" not full blackpowder power level loads, though it is possible.
I use 10gr under a 400gr cast slug, don't know, and don't care how fast its going. though I am certain it is subsonic. Its not a hunting load, just a light plinker for fun use.
Many will tell you that kind of load (and your load of 15gr) should use a filler of some kind. I won't disagree, for field use. The powder does need to be back next to the primer. I don't use a filler, since mine are plinkers, (and fired from single shots) I simply point the muzzle straight up after chambering the round and then lower it down to level to aim and shoot. I've never had a problem but can't say you won't, so I don't recommend doing what I do to anyone else.
I wouldn't waste a jacketed bullet in any load moving less than 1200fps, but its your money.