Your blue LEDs need about 3.5V. The current-limiting resistors need about 1V. The astable transistor circuit needs an additional 1V so the minimum battery voltage is 3.5V+1V+1V= 5.5V. Four Ni-MH rechargable cells will give 5V (not enough) and five will give 6.25V for quite a while. They would need 7V or more from the solar panel to charge them.
I've been reading this thread for a while now, and I got more and more
confused because the LM3909 never showed up. I quote the datasheet:
" The LM3909 is a monolitic oscillator specifically designed to flash Light
Emitting Diodes." I tried Google and got 38600 hits, I tried my local shop
and found that it was still available (and cheap). My conclusion is that
there must be something wrong with me !