Blacklights tend to be in the UVA region (least harmful), as the wavelengths get shorter the damage threshold is less. Anything in the UVB or C catagory is the most harmful, you were specifically talking about 266nm which is within the UVC catagory and is therefore the most harmful.
Cataracts usually form over a period of many many years of exposure to low dose UV radiation from the sun. This could obviosly be accelerated by exposure to 'artificial' UV. UV isn't the only thing proven to cause cataracts, try googling cataracts and smoking
The following is from some technical info I had kicking around, i'm sorry, but I don't know the author, this explains it fairly well...
Ultra Violet (UV) light represents a section of the overall electromagnetic spectrum of light, extending from the blue end of the visible (400nm) to the x-ray region (100nm).
It is subdivided into three distinct wavelength regions described as either UV-A, UV-B or UV-C in increasing order of photon energy.
UV-A 400nm-315nm: Often referred to as 'blacklight', this is the longest wavelength region and lowest energy, it represents the largest portion of natural UV light.
UV-B 315nm-280nm: Partially blocked by the ozone layer this is the most aggressive component of natural UV light and largely responsible for sunburn (erythema).
UV-C 280nm-100nm: Only generally encountered from artificial light sources since it is totally absorbed by the earth's atmosphere.
Cheers
Jem