Re: A writeable CD left on window sill
I use properly stored Blu-Ray discs as well but opt for using rewritable discs rather than write-once. Write-once discs tend to be based on dye staining/burning which over time can fade.
Rewritable discs use a phase change medium that physical changes the layout and should technically last longer. The closest analogue I can find is "It melts the damn disc". It also allows updating or refreshing the disc by rewriting it again.
Personally I find backup to only have two options at this moment. Tape which is large, removable, robust. Optical which is smaller, removable and mostly robust.
Hard drives are not a good alternative for storing long term archive data. Some hard drives will even seize after standing for a very very long time.