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IME a Barlow never improves the optics of the eyepiece. It can however reduce any visible off-axis aberrations simply by virtue that the longer light cone produced by the Barlow makes it easier on the eyepiece. It can also make the eye relief of the eyepiece a little longer, usually arounf 20-40% longer in my tests. This can be good if the eyepiece has short ER, and bad if the eyeppiece already has too long ER that makes it difficult to use.
For poorer quality Barlows they usually add noticable scatter to the view and can commonly add lateral color also. Good quality ones can be very transparent and not take anything away from the original eyepiece's performance. I find the Baader VIP and Baader Zeiss Barlows, and the APM 2.7x ED Barlow to be in this class.
The Barlow is a pretty handy tool. I keep and use several. But rarely use them for planetary observing since that I like to keep as pure and with the least scatter possible.
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