"..you wouldn’t refer to them as ‘darling’ when speaking about them to someone else."

Brilliant, that's it.

Yes, I'd agree with Dee and Kate - 'darling' isn't actually a name in this context as Mum or Gran would be. You wouldn't write 'Morning, Gorgeous!' would you? Except I can imagine doing so in a very light-hearted novel where you were playing games. But the joke would be that it is a game - it's not a name.

Emma

<Added>

You've nailed it perfectly, as usual, Dee.

Hi Dee,
OK, here are some other horrors:
'Come here you lovely girl.' a description.
'Come here Lovely Girl.' a name or a description, if description is it 'Come here, lovely girl.'?

'You look life-like now, my Jenny.' [her name]
'You look life-like now, my Lamb.' [the new name he's given her]

And what about 'Like an eye on the ocean, Son.' [Son is not his name, but his mother calls him that].

I so don't want to do more proof-reading, but if I must, I must. Lol!
Becca.

I was just thinking that Sonny/sonny is a good example…

Freda sees Johnny in her garden. She shouts, ‘Get off my lawn, sonny.’ Later, telling a friend about it, she’d say, ‘I told Johnny to get off my lawn.’ She wouldn’t say, ‘I told sonny to get off my lawn.’ ‘sonny’, in this context is used as a term of endearment (in a rather unendearing way, I’ll admit), and the friend more than likely wouldn’t know who she was talking about, so sonny shouldn’t be capitalised here.

If however, the kid’s real name is Johnny but everyone – friends, relatives, anyone who knows him – calls him Sonny, then Sonny is a nickname and Freda would say to her friend, ‘I told Sonny to get off my lawn.’ The friend would know exactly which boy Freda was talking about, so Sonny should be capitalised in this case.

Dee

Hi Dee,
thanks, I think I'm clearer now on which to capitalise and which not. I saw a piece of dialogue with darling without a capital d, so I think I'm clued up now, and thanks to all who replied, very useful.
Becca.