My black dog flumped onto my feet while I watched television, formed to the curvature of the couch, and in his mouth was the tattered remnants of my red balloon.
All afternoon I had watched him skulking around the back door, but he nuzzled his way in, eyeing off the red balloon that floated on the draughts through the cracks in the windows.
With a quick snap he had taken the balloon in his mouth and popped it.
He looked triumphant with his saggy jowls; that I was defeated and would not move from where I had taken up position.
The afternoon shadows crept like soul’s darkness across the floor, a marshy quicksand that sucks you down below into its depths.
However, my black dog tends to forget that I have a pocketful of red balloons.
A Fullness in Brevity - Adam Byatt 
The last in the Red Balloon series.
The Black Dog is a recognised metaphor for those suffering from depression, poularised by Winston Churchill, amongst others.
I’ve had this one percolating for a while, but needed a final line.
The red balloon has always been about hope, and even in the darkest moments, there is hope.
This is for all those I know who suffer from depression.
http://www.blackdoginstitute.org.au
Nice! I’m very familiar with Churchill’s black dog metaphor, but still had to read this twice to understand the meaning, because I tend to take the black dog at first glance as a symbol of power and protection… and the only time I’ve come across the symbol of a red balloon is, well, “99 Luftballons”. Which isn’t entirely a message of hope. 😉
Despite that, the intended meaning was clear with a little thought!
Tangentially, black dog apparitions and ‘ghost dogs’, relatively common in my country of origin, are a favourite folkloreish topic of mine, so I tend to read anything involving them.
I really liked this, especially the last part about having a pocket full of red balloons. Hope floats!