[1eyedjack]

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Or maybe not (go back to opening 1N - see "umm" thread)

South is a bit top-heavy for 1N (the shape does not feature), but oddly enough we were on course for the top spot of 6C, until North passes 5C.

I have a feeling that this has been raised before, but I thought it had been fixed. Evidently not.
We are often in a world of hurt with 4N being RKCB in Clubs, there being only one response that does not commit to slam. But OK, we have to suck it up and live with it.

It seems to me that when Blackwood responder gives an ambiguous response, then Blackwood bidder should often take a pessimistic view (0 rather then 3, or 1 rather than 4). Blackwood responder would then reject partner's signout when holding the stronger holding. But that does require that Blackwood responder is granted another bid, so that rule only applies when Blackwood bidder is signing out by bidding rather than by passing.

If the response to Blackwood is in the trump suit, then you just have to guess. And here it is not a difficult guess.
The strongest hand that South could have consistent with zero key cards is ♠J, ♥KQJ, ♦J, ♣QJ. That is only 11 HCP and not sufficient for a 1N opener. So South is known to have 3 key cards. There being only one missing key card, and that is sufficient for North to commit.

Another solution is for some higher response than 5 of the trump suit to be coded to show the stronger of the ambiguous holdings that would otherwise have required 5 of the trump suit. That would be a safe solution, but has it been coded? I don't think so.