I've specifically mentioned that my results were for DW Frost. For 2h Frost, yes: Runic Empowerment is likely better or at least very close to Blood Tap.
For DW, what's the best way to keep an Unholy rune on cooldown? I'm currently doing D&D > OB if no KM > Plague Strike if Blood Plague about to expire > OB with KM. My goal is to never have 2 Unholy runes up, and to waste the least number KM procs and Frost / Death runes on OB while doing so.
I usually save my BT for when I immediately need the rune, like to HB for RP to FS when KM procs.
Any additional target and/or movement immediately skews that balance in favour of HB/DnD.
Also, while KM FS is obviously more preferable over KM OB, do not hold back KM procs just for the sake of building up the necessary RP, use the proc before it's overwritten.
Euliat's already explained why this is the way to play.
The reason you "gamed" Unholy runes with Runic Empowerment was to force Death and Frost runes to proc and grant more Howling Blasts.
With the way BT works, you do the reverse: If all runes are on CD, Blood Tap will currently ALWAYS return an Unholy rune as a Death Rune before going back to a F/D rune, meaning more HBs per rune cycle.
Therefore, you have strong incentive to ensure both Unholy runes are on CD as often as possible. It also goes without saying that spending more runes (instead of holding them back) = More RP = More Frost Strikes = More BT procs.