I've been traveling by bike in eastern Brandenburg, Germany and have encountered at least 4 of those. My guess that it was somehow connected to camping or maybe boats? Couldn't find anything online, searching for
blue cross is pretty hard.
They are probably to signal political agreement with some causes.
For example, blue crosses are/were used to protest for free public access to waterbanks without private fences in eastern Brandenburg (which is probably what you've seen) and against the building of the Fehmarnbelttunnel in northern Germany.