Depends on the tape. A nice light-absorbant black tape will just make the tube very hot. A suitable reflective tape could potentially reflect light downwards, but it would be directed back into the tube and that is not very efficient. Some of the light will go through but some of it will just heat up the tube. You can see how much light would go through by shining a flashlight through an off tube. I wouldn't have though this was a good solution.

An external reflector reflects most of the light around the tube rather than through it. Much better idea.

You got it with the incandescents, they are a clear, nearly empty, glass bulb. At worst they are frosted.

You can always experiment with the taped tubes and see how much brighter they are. At the very best, they would direct all the light that would have gone up back down, but the tube would still be acting as a point source and spraying light out over 180 degrees. A curved external reflector will direct the majority of the light into a much smaller downward angle, which is better for intensly lighting plants.

just been reading all these posts on how complicated adding lighting can be for some and thought I'd bring it up.
SOUNDS TO ME FROM YOUR INFORMATIVE POST THAT FOLKS WHO HAVE NO INTENTIONS/WILL/DESIRE/WAY TO BEND A CHEAP REFLECTOR,

IT'S BETTER THAN NOTHING THOUGH MY ELECTRICAL ENGINEER FATHER DISAGREED WITH ME AS WELL WHEN I ASKED HIS THOUGHTS A FEW YEARS BACK.

as a side note, here is a pic of my girlfriend's fridge and the bulb i am taping up. She calls this setup "Ghetto" LOL this will shut her up, or make it even more "ghetto" lol again!

Here is a link that might be useful: {{gwi:1022464}}

btw dont go buying "lights of america bulbs" they have a horrible reputation.
i happen to have a truckload of these things I got from a family friend who is a freelance retail lighting rep here in NY.
I have a ton of good, worthwhile stuff as well..A 2 car garage full of electrical supply/lighting came to me after his father died and had to sell his house. (email me for trades for seeds, i am very generous providing you pay UPS ground shipping)

A good/better reflector is a better choice than taping,IMO.

I already established your point a few posts up.
let me stress this again,
IT'S BETTER THAN NOTHING.