I don’t know what the flashing means, but I also sometimes see that with a third-party charger, though generally at the end rather than the start of a charge, and if I unplug it and plug it in again it reverts to the normal steady blue. If that isn’t happening with yours then you’ll probably need to try a different charger, though it could of course be the instrument that’s faulty. Artiphon advise against using third-party chargers, though I think that’s largely because they’re worried about people using the wrong voltage (it needs to be 12v or it won’t work and could damage your instrument). Check that the specs on your adapter look something like this: