Fearing they might have been struck with a bizarre illness after cycling through a forest in Puglia, the pair immediately took themselves off to hospital.
But doctors were equally mystified by the blue marks, which could not be washed off, La Repubblica reported.
The cyclists thought it might have been caused by pesticides used in the Mercadante Forest in their town of Cassano delle Murge.
It took a toxicologist from the University of Bari to finally get to the bottom of the matter: the problem was caused by a substance from the blue sports tops they were both wearing, which was not toxic but could still cause moderate irritation to skin.
The cyclists are now mulling whether to pursue legal action to find out if the garments, made in China, were in line with European health and safety regulations.
It is not known if the tops underwent the checks required for CE marking, which identifies products as having passed the requirements set out in EU legislation.