I found in an older connectX-2 pdf this statement.
Blinking indicates a problem with the Physical link
Which is odd for me as it does this with the SFP plugged in only no cable connected. This is different from the behaviour of the other port. When I move the SFP with no cable plugged in into it there is no light at all, and of course it is find with the cable plugged in.
The other port blinks with or without cable - the exact same cable that works in the other port........