Over the past 18 months i have noticed that a lot of twitter bootstrap admin and other themes with left nav bars are now using a black or dark blue colour.
e.g. https://wrapbootstrap.com/themes/admin
I have also seen other sites such as Quickbooks, Salesforce & Microsoft Azure adopt this colour scheme.
What benefit does having a darker colour give over having a lighter one? Is it just to make the website look more fancy or does it improve usability at all?