DIAGNOSIS A blinking blue light means the Deco is booting up. If it's stuck like this after an outage, the boot process may have been interrupted, or it's having trouble getting an IP address from your modem.
FIX You need to power-cycle everything in the correct order. 1) Unplug the power from your modem, your main Deco, and all satellites. 2) Wait a full 60 seconds. This clears everything out. 3) Plug only the modem back in. Wait for all its lights to settle into their normal 'online' state. This can take 2-3 minutes. 4) Now, plug only the main Deco (the one connected to the modem via Ethernet) back in. Wait. The light will cycle and should eventually turn solid blue. 5) Only after the main Deco is solid blue should you plug the satellite units back in, one by one.
IF THAT DIDN'T WORK If the main Deco light goes from blinking blue to solid red, it means it can't get internet from the modem. At this point, unplug the Ethernet cable from the Deco, wait 10 seconds, and plug it back in firmly. If it stays red, try a different Ethernet cable between the modem and Deco. The outage might have spiked the modem, requiring a full factory reset of the modem itself (check your modem's manual for that).
HEADS UP The order matters. The modem must be fully up and happy before the Deco tries to talk to it. Rushing the steps is the most common reason folks get stuck in a blinking loop.