How would you translate “About that...” in the context of regretting that something happened or not wanting to admit something related to the topic at hand?
“I’m going to bring my new blender to the neighbor’s to let them borrow it. Could you grab it for me?”
“About that... I accidentally broke it.”
or:
“I can’t wait to go on this date... he’s perfect!”
“Yeah, about that... I saw him kissing someone else yesterday.”
I'd say À ce propos... or À ce sujet... (I'd use more naturally the former)
It's quite usual in French and conveys the same idea of embarrassment as in English (contrary to the simple Au fait which is neutral). And it is quite a similar construction as in English, so easy to remember.
In the first case "Ça alors !" can be used among other locutions ;
Ça alors ! Par maladresse je viens de le casser …
Zut (alors) ! Je viens de le casser en n'y faisant pas assez attention. informal, but current
In the second "ça alors" won't do ; here are two possibilities among a whole lot of them ;
Oh ! oui, en parlant de perfection … Je l'ai vu hier en train d'embrasser quelqu'un d'autre.
Oui, en effet, ô combien parfait, je l'ai vu hier en train d'embrasser quelqu'un d'autre.
ça alors might be used in the same situation but it conveys surprise, not the same feeling of regret or embarrassment as About that. It is more an equivalent of My goodness!. For the second sentence, your en parlant de... is a nice idea, but Combien parfait seems awkward.