OK, so you've got your LED's, plus some way of controlling them, ergo a darlington transistor.
First things first, if the battery on your bike is 12V, unless you want to get into the weird world of switched mode power supplies then thats your maximum voltage. This means that if you put LED's in series, you're looking at a maximum of 3 + current limited reisistor. I suspect the forward voltage of most blue LED's is >3V so to be safe I would put two LED's in series, along with a resistor.
The value of the resistor you choose depends on the voltage across the LED's, the voltage of your bike battery (12V but it WILL vary quite a bit) and how bright you want the LED's to be. Some larger LED's can take up to 300mA, but the smaller 5mm ones will take about 20mA. I wouldn't go over 20mA.
Heres a quick example.... put two led's in series, along with a 330-680 ohm resistor. measure the voltage across the resistor. Forget about how bright the LED's look for now, if they are bright, then thats fine, if not, then you can change the resistor value.
Resistor value = voltage across resistor/ current you want for LED's.
So for 20mA (15mA would extend the LED life) R = Vr/0.02.
here's a quick schem I've drawn up so I don't go on forever explaining bits. Its a 'rough' schematic, many values left out, just so you know the sort of thing you'd need, and how everything fits together.
Thats the 'true' DIY approach. There are other ways, like buying a premade thing for flashing LED's and modding it. 30-50 LED's is quite a lot, considering many of them are narrow viewing angle, that means you'll be kicking out a hell of a lot of light in one direction.
If you need anything else, just ask.
Bleteeth
Edit: I'm an idiot....12v/3 = 4. You could have THREE LED's in series. You'd need a lower value resistor. Lower power disappation in the resistors and transistor for the given number of LED's.