UKH

Anyone ever found blue spots up their back after a road ride? Twice now alongside the usual mud spots I've also found some unusual blue spots. Not dissimilar to someone flicking a fountain pen at school!

Any ideas what it's from? I guess some sort of road contaminant?

Are you running tubeless tyres? If so I’d bet you have a puncture that’s not sealed and you’re losing sealant.

Nope, good old tubes

How about bird poo being thrown up off the roads by the back wheel from birds that have been eating blueberries ?

Do you mean on the back of your jersey, or actually on your skin?

You didn't ride over a fountain pen, I assume?

That could be cyclists-blue-spot, a rare condition that results in multiple bikes and eventual divorce.  Have a motorist check you out.

Back of jersey, two separate occasions, different routes and different bikes. No pens were harmed.

Dye from a helmet dripping coloured sweat down your back? 

Quink-Step by proxy?

Odd it's on 2 routes - I was going to suggest riding the same route again and frequently checking the back of your top for blue spots. Perhaps it's closer to home - something on the floor of your garage or your road?

https://www.flickr.com/photos/77473778@N00/4213139125/

Maybe it's contagious...

I noticed the same thing on Richard Carapaz at last year's Vuelta:

https://img.aso.fr/core_app/img-cycling-vue-jpg/cl-vuelta22-etapa21-99/1607...