> It looks right.  Please install.

It was too simplistic.  In fact I have to distinguish two cases when
`blink-matching-paren-distance' is non-nil:

- `point-min' is at most `blink-matching-paren-distance' characters
   before `point'.  If in this case I don't find a matching paren there
   cannot be any such paren and I should treat this case just as with
   `blink-matching-paren-distance' nil.

- `point-min' is more than `blink-matching-paren-distance' characters
   before `point'.  If I don't find a matching paren now there could
   still be one before the start of the narrowed region.  In this case
   I'd have to display a different message (or maybe no message at all).

See the attached patch.