Black upon black

Hi in the following poem what do you think is this expression at the end of the poem black upon black
BIRD
When I became a bird, Lord, nothing could not stop me.

The air feathered
as I knelt
by my open window for the charm –
black on gold,
last star of the dawn.

Singing, they came:
throstles, jenny wrens,
jack squalors swinging their anchors through the clouds.

My heart beat like a wing.

I shed my nightdress to the drowning arms of the dark,
my shoes to the sun’s widening mouth.

Bared,
I found my bones hollowing to slender pipes,
my shoulder blades tufting down.
I   spread    my flight-greedy arms
to watch my fingers jewelling like ten hummingbirds,
my feet callousing to knuckly claws.
As my lips calcified to a hooked kiss

Silence

then an exultation of larks filled the clouds
and, in my mother’s voice, chorused:
         Tek flight, chick, goo far fer the Winter.

So I left girlhood behind me like a blue egg
and stepped off
from the window ledge.

How light I was

as they lifted me up from Wren’s Nest
bore me over the edgelands of concrete and coal.

I saw my grandmother waving up from her fode,
looped
the infant school and factory,
let the zephrs carry me       out to the coast.

Lunars I flew

battered and tuneless

the storms turned me insideout like a fury,
there wasn’t one small part of my body didn’t bawl.

Until I felt it at last          the rush of squall thrilling my wing
and I knew my voice
was no longer words but song       black upon black.

I raised my throat to the wind
Thanks