All I use butyl tape for under the lights is to plug the hole where the wire goes in. It depends on how the light is made. Some have solid backs, others are just a shell.
The main reasons I don't use silicone, is because once you use it, it is all but impossible to completely remove and nothing will stick to it, including new silicone. It will also "microfracture" and lead to leaks in the sealant itself. It is a good product for something like windows in a building that have minimal movement, possibly some uses in a bathroom. The only place I have used it recently was when I had to splice in some LVP flooring and that is what the manufacturer called for. Not sure what your black tape is, all the butyl I have every dealt with has been gray.
Aaron