How's this?
The IC is a 40106, a hex Schmitt trigger inverter.
I have assumed that you want the device to power up un-triggered.
R1/C1 ensure that the input to the first gate is low when power is applied. S1 acts as a reset switch to pull it low again.
R5 is a CDS cell, I have assumed you want the device to turn on when it sees light. If you want to use a switch, replace this with a switch that is closed to trigger the flashing. Also place a low value resistor in series (say 1K) so that pressing the reset switch with this activated doesn't short out your battery.
The first 2 gates with resistor R2 form a latch. When the input to the first gate is pulled high, the output goes high and stays high until the input is pulled quite strongly low. The CDS (R5), reset switch (S1), and the capacitor (C1) are all capable of doing that (In the case of C1, it can only do it for a brief instant after power is applied. Resistor R1 is NOT capable of pulling it low.
The third gate is simply a buffer so the next stage doesn't disrupt the latch.
Diode D1 allows C2 to charge from the output of the third gate, but not the discharge. When the output of the third gate is high, the 4th gate cannot oscillate. When the output of the third gate goes low, the 4th gate will start to oscillate. The frequency is determined by R3 and C2, and the frequency is about 1/(0.6*R*C) from memory.
The fifth and sixth gates buffer the output of the oscillator and provide extra current drive to the LED.
When it is first turned on, C1 looks like a short term short circuit, so the input of the first gate is low. Thus the ouptut is high, and the output of the second gate is low. The output of the second gate feeds back to the first gate via resistor R2 which maintains this state.
Because the output of the second gate is low, the output of the third gate is high and that charges the capacitor C2. Resistor R3 (which should be in the order of 10K to 1M) prevents the output of he 4th gate from overpowering the third gate, so its output remains low.
the inputs of the 5th and 6th gates are low, so their output is high, and thus the LED remains off.
When the CDS (R5) is exposed to strong light, it provides enough current to overpower R2, pulling the input of the first gate high. The output of the first gate is low, the output of the second gate is high, and this feeds back to the input of the first gate holding it high against R1.
The input of the 3rd gate is high, so the output goes low. The diode D1 effectively disconnects it from C2.
Now resistor R3 begins to discharge C2. At dome point the change is low enough that the schmitt trigger changes state and the capacitor begins to charge via R3. This goes on until the schmitt trigger changes state again... The gate is oscillating.
These oscillations are fed to the inputs of gates 5 and 6. Their outputs then flash the LED.
When S1 is closed,the input of the first gate is pulled low, and everything progresses as per the beginning causing the LED to stop flashing.