I avoid their petrol, only run V power, so i am always a little dubious. I have used their fully synthetic oils years back with no issue. As long as meets correct specs, strictly speaking it should be fine. I know some manufacturers add their own additives (maybe like KFC with their recipe lol). But they also hold your wallet to ransom.
With me doing regular changes, i dont really want to pay for longlife oil if makes sense. The newer oils for longer service intervals is all fine, but my oil will still be changed every 3k ish. So i am not sure what extra value i would get, the v8 has just turned 110k, previous oil was at 107k. I will be honest, i do research things to death sometimes, end up burning my head out. As you can find too much info. Now that oil i have in right now, they do a 229.5 which is for long life. But at 3k intervals, is it really important? I can buy 10L of 229.3 for a little under £45. The 229.5 comes in 4L tubs, so the cost jumps to £90 for 12L(buying 2x 1L at £9.99 + 2x £30 would still cost £90 ).
Not heard of blackstone, no, a quick looks shows they test oil samples/ analysis.