No, it's a Kentish saying then I expect

......seeing as it was misunderstood:

Whilst I was whinging the other day about the osteoarthritis, Westerwitch said that she has sjogrens. Sjogrens is nasty, and I said so, adding " I've got no right to complain ", but then
Westerwitch said to me: " I did not mean to upstage you" and I replied: " I don't believe that you have a black cat "

Now are we all following this so far?
I know it gets complicated with the ' I said, she said, and she said, so I said...' stuff.

Anyway consider the person who has a bigger problem than yours ( regardless of the size of yours )
or the person who got a better bargain on their new ( whatever ) than you did
or the person who is more ill than you are
or whose children did better at school than yours
or if you've got a black cat - their's is blacker than yours is!
So if somebody is being in any way ' upstageful ' we ask what colour their cat is.
So when I tell Westerwitch she doesn't have a black cat, I don't mean she's telling lies about the colour of her cat - I mean I know it wasn't a ' degree of pain ' competition.

Regional variations abound, no doubt. On the subject of which: I thought we'd agreed to leave the rain dancing??? It's looking a bit black over Wilf's mother's.