What would happen if when BB was in the middle of one of her negative rants you just told her "Quit whining you miserable cow". Much in the same fasion MJ called her H a fastidious prick. I am guessing she would up the ante which is why it is something you would shy away from doing, but in some ways I see it as a simpler solution to a) blowing it off or b) gently correcting her. The one thing about this is that it is HONEST. It is an honest expression of your feelings when she starts being negative. It also doesn't invalidate her opinion it just shows her that expressing it comes off negative and whiny.
Just my 2 cents
Fran
p.s. you asked me about why we don't have a/c here. It's mostly because there just aren't that many really hot months in the year. Most of the housing stock is Victorian so the houses are solid brick and have high ceilings which means they stay reasonably cool in the summer. It's just when the temperatures soar up into the high 80s or even 90s (celsius) that it gets unbearable. But like I say it is rare. Also I still haven't dug out a bill to let you know the price of power per KW/h but I'll bet it's at least twice what you guys pay. All our energy costs are much higher here.
if we can be sufficient to ourselves, we need fear no entangling webs Erica Jong