My Raspberry Pi B+ had a blinking red LED whenever I connected it to the USB ports on the Asus RT-N66C router. Because I've read everywhere that it meant that it was experiencing a brownout, I installed a DROK USB tester inline (it shows voltage and current on the USB connection). The connection voltage was a constant 5.01 V and current 0.23 A.
As this was not a condition for brownout, I gathered from several answers on the Raspberry Pi formus that the cable type had something to do. My guess was that it shouldn't change, right?
I swapped the cable I was using, that only had charging leads active, with a full data+charge cable, and the LED stopped blinking. Current and voltage did not change, so I suspect that the B+ power supply somehow mistakes the lack of data cables as a low-voltage situation.
This also explains forum answers that say that changing cables sometimes makes the blinking go away. Nowadays, some USB cables are used for charging only, and do not support data transfers. The B+ expects all leads to be supported. Hope this helps!