The phrase 'blue flu' is more appropriately applied to a form of unofficial police — hence 'blue' — industrial action, where simultaneously most of the force calls in sick. It's featured in one of the Transmetropolitan books, and I think it occured in an episode of one of the CSI shows, IIRC. Or was it The Wire?

I don't know what you'd properly call it in the case of dockers: 'brown flu'? 'Lumberjack-shirt flu'?