Over here dark fiber has meant private, dedicated fiber since the '90s. As opposed to connections managed by your telco. Seems similar in the US per https://en.

Over here dark fiber has meant private, dedicated fiber since the '90s. As opposed to connections managed by your telco. Seems similar in the US per https://en.wikipedia.org/wiki/Dark_fibre https://en.wikipedia.org/wiki/Dark_fibre