There is an assumption that plants do not need much nitrogen after switching to flowering. In actual fact in the first 3 weeks after switching, some plants can consume almost all the nitrogen they have in stores. In particular plants that put on growth spurts that can result in them almost doubling in size after switching. That 'doubling' in size is done via the macro nitrogen.
You should try to find a balanced nutrient that has the full range of macros (NPK) and micros something like a 10-10-10 or 20-20-20 and feed them with that until week 4 at which point you can start on the 10-15-10 type formulas because plants generally do not start to demand P and K until after week 4. Prior to that they need a more balanced NPK ratio.
Your plants are in need of nitrogen, so my guess is two things, one is a little bit of over watering and that the bloom forumula you're using is probably promoting phosphorous to dominate the rootzone too early thus not allowing any nitrogen in there is to get into the plants systems. That will cause the nutrient burn you are seeing, a buildup of phosphorous due to the plants not really needing a lot of it at this stage in their dev.
So they simply just leave most of it behind in the pot.