Don't know much about the Juwel specifics of getting replacements for your tubes, but, I do think Juwel tubes are good.
There's a site here that seems to sell either Juwel or at least compatibles...
http
/www.iquaticsonline.co.uk/juwel-compatible-t5-fluorescents/54w-t5-1200mm-48.html
As for what you need it's very much down to personal preference. If you have no plants at all you don't need to worry about wattage / litre and all that stuff. All your light needs to do is show off the contents of your tank. In my opinion, there's nothing better than a good powerful, full spectrum white light.
There might be a couple of ways to achieve your blueness...
Getting 1 or 2 blue tubes with 2 whites is one way if you want a strong blue hue in there. The other way and perhaps a little more subtle is to get a really high K colour of tube, I'm sure there's one on that site rated at 10,000K. Without getting too technical, a high K value is in the blue/cold end of the spectrum of light, low K values are warmer, redder tones. About 4000K - 5000K is about in the middle and as close to white as is humanly perceivable.
If you stuck to high K rated tubes you'd get a nice cold/blue tone in your tank. I'd say definitely get 2 proper white tubes though...the actual set up and configuration of them would be hard to recommend without seeing the effect 1st hand so it'll be a trial and error thing to get it how you like it!