Adding Blinking LED to Low Battery Circuit
You can use a schmitt-trigger nand gate (in my experience most nand gates have schmitt-trigger input) and couple the output through an RC circuit to one of the inputs and use the other input to activate/enable it. the oscilation frequency will be 1/RC because (most) nand gates are set to have thresholds so that the charge/discarge time between thesshold voltages will be tau. there are many other ways to make an oscilating circuit but this is one of the simpler.

to detect the voltage level you could simply use an op-amp.

If you want to make a low battery indicator that behaves this way what I would do is I would use a small 8-bit mcu like an atmega328 for instance.

The atmega328p if I remember correctly can go down to almost 2v of supply voltage if you give it a low enough clock frequency (see the datasheet) and it would probably have a smaller footprint than your analog circuit.