I happen to have a saved game right before London gets blinking trade cash (in addition to blinking growth from trade, and blinking health). It is in the amount of 508 or so IIRC (I noted the difference between the blinking total and the non-blinking one when highlighted). This amount corresponds to the amount of one of its sea trade routes. However, when I end the turn, that route isn't gone... in fact nothing noticeable happens to the trade route, as displayed on the map or on the details screen. What I did notice is an event that seemed to cause the blinking - London got a new import route on that turn. Coincidentally, the growth and health bonuses weren't ending that turn either - they were new from that new incoming trade route. So in light of that observation, my conjecture is that the blinking is not intended to tell you of something that is ending, but rather something that is beginning, just from a source that is not the city whose detail scroll you're currently looking at. In my case London gained income because of a sea trade route building completing somewhere else, so London gets blinking trade income icons. I still have no idea why it would be in the amount of the trade route though - maybe the import route was from the same city London had been exporting too already, and that caused it to blink. I'll have to check, and also fiddle around with it more to see if what I've notice is really what is going on.