I'm guessing that you've removed the infrared and UV (lowpass) filters from your camera?
The purpose of those filters is to limit the light hitting the sensor to be predominantly in the human visibility spectrum. Without those filters, you have light wavelengths punching through the nominal Red/Green/Blue¹ filters on the sensor elements and activating them to produce False Color, that is UV will activate the RGB sensors at various levels to produce varying levels of red/green/blue for the UV light. This is a remapping or False Color for UV. The same happens for Infrared.
The first thing this means is that the very concept of White Balance no longer exists. White is a human physiological interpretation of a mix of the visible spectrum. Your modified camera is detecting outside of the human visible spectrum, so white no longer has meaning. This also means your auto exposure system will be whacked out.
JPGs are processed from the raw sensor data with programmed assumptions about the spectrum detected. You've altered that spectrum and invalidated the program assumptions.
Ultimately this means that except for a few blind luck shots, you really need to be working with RAW to have your best chance of producing an attractive false color photo.
As to the noise; the Blue sensors constitute only 1/4 of the sensor array and UV will likely dominate here.
¹ For the overly pedantic, yes Red/Green/Blue filters are not perfect optical impulse functions and have their own response curves.