Although tape on the OUTSIDE of the enclosure has very minimal risk, I personally think there are better ways to black out tubs. I personally use black spray paint (the kind used for plastic/outdoor use). I spray the outsides of the sides, back and lid and let it all air dry for at least 3 days or so and will usually give it a good rinse and dry before putting any snakes in there.
People very often use Con-tact paper to black out the sides and backs of tanks and such, I really don't see how that's any different from duct tape. Like some have said as long as the snake can't roll around on it the risk is minimal. I think if you personally would never do it, then that's fine, but state it that way. Don't try and make it sound like it's a "dead snake walking" if you use an adhesive on the OUTSIDE of an enclosure to scare newbies.