The power LED you are referring to is actually the status power led itself, which in normal sutuation should either be off is the system is switched off from power supply or on if power is connected to the system.
But in your case, it is blinking red, meaning there could be something wrong with the power supply.
Also since you have tried with another similar motherboard and it's the same, then it clearly indicates the power could be the cause.
Make sure your are using power supply with satisfy the PSU requirements:
Also, it is recommended to have a power supply with at least ATX Version 2.2 and for this board at least 300W or more if you are using more memory, add-on card and other components.
Thanks. I replaced the 450W PSU with a 650W Antec Tru-power. I've never noticed the red led before, just the green one among the capacitors near the edge of the board. Now the both come on when I trurn the psu on. The red goes out after a few seconds. When I power-up, both lights are on steady. I understand you correctly, this is as it should be.