The phrase "squid ink" refers to the black ink that squid produce. It is often used metaphorically to describe something dark, mysterious, or secretive.
Sentences with «squid ink»
«Hanging in the tokonomaalcove was a single abstract painting, painted with squid ink on Japanese paper. (michaelwerner.com)
I loved the contrast in colours between the black squid ink risotto and the delicate prawn and grilled octopus resting on the top. (london-unattached.com)