I don't think a blacklight will hurt anything... I'm pretty sure the sun emits the same color in some spectrum or another. I'd say the reason they're turning brown is from insufficient lighting. A 50/50 bulb only provides a little bit of usable light. I'd swap it out for a true daylight bulb (something in the 6500k-10,000k range). The wattage and type of bulb will have some play in what plants you can keep as well.
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