What is this black stuff
Your yew has an infestation of an insect known as cottony camellia scale. Scale insects have a protective "shell" or covering. See the little brown ovals on the needles in the first, closest, photo? Those are the young scales.
They suck nutrients out of the leaves, then excrete a sweet liquid known as honeydew. It falls n leaves/needles below them. In the summer, the honeydew gets a fungal growth that is black, that's called sooty mold. The sooty mold is the black substance you are seeing, but it is just a by-product of the scale insects.
You can spray your yews with a dormant oil now, and a horticultural oil or soap in the summer. Here's what you need to know: https://extension.umd.edu/hgic/topics/cottony-camellia-scale-shrubs