It is well-established that GMCs are the clouds in which star formation occurs. Sites of massive star formation. Associated with spiral arms and GMC .'s. These are red, glowing pictures that you see in the text. One sees the reflection of blue light by the dust particles in the cloud, i.e., the light from a hot star is reflected by the dust in the cloud.
Dark clouds are regions of high dust concentration.
The dust very effectively blocks out the stars which sit behind them making what appears to be a hole in the sky .