I'm not sure how penetration works in 4th edition, but in fifth, it's a multiplier on your penetration skill. Blood doesn't auto-bypass parma, no, it helps however.
This is the single most important improvement in 5ed IMO. Throughout 3ed and 4ed I could never understand why Parma Magica could have been so instrumental in the foundation of the Order, while in fact it sucks. Any reasonably mature wizard can get through your MR with their most powerful (or any other) spells. (Neither did Parma protect against the effects of the Gift in 3ed, and I don't think in 4ed.)
In 4ed the AC was hardly necessary, and I do not remember any rules for using AC to penetrate more easily. In 5ed, this is half-way true. An AC on its own is not particularly good, and neither is the penetration skill. The combination of AC and skill, in contrast, is powerful because they are multiplied, and this, I suppose, explains why magi are so careful about leaving ACs behind.
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That's in The Mysteries, "Penetration Mastery", for 4th ed. Which is why I so readily accepted your suggestion of modifying the penetration rule for targeting, they're simply two variants of the same rule IMO. That AC penetration rule seems to also be common, at least in part, to both 4th and 5th.
I will also point out as the OP that the question was fundamentally about 5th ed.
I believe the same solution I jumped on was originally suggested for use in 5th ed., though @loke didn't specify.
I didn't, and I do not think it matters, but you are right that my mind is tuned to 5ed.
For the record, I won't advocate that «suggestion». I was merely spinning ideas to explore the original question and possible implications. @temprobe explained why it is contrary to the fundamental principles of magic, penetration, and finesse, and he is right.