Do
not update portage using portage, it could be dangerous.
Use entropy to update portage.
The version you want to install does not appear because you unmasked the 9999 version.
remask that version, removing the entries for the 9999 version in the:
/etc/portage/package.unmaskfile, eventually, remove also the entries in the:
/etc/portage/package.keywordsfile.
The 0.9.1 versions are not masked.
It also looks like you have a darktable in your local overlay (and that you have the sabayon overlay in
/usr/local/portage/layman/sabayon 
), why would you want it? It is available on portage, it would be better for you to remove the ebuild in your local overlay (and to put a repo_name for it so that it will stop complaining for it

).
Once you did this, you could proceed as follows:
1) try to minimize the use of portage installing the dependencies using equo:
(as root)
- Code: Select all
equo install lensfun cmake
2) Now you should change the USE mask for
libraw, you have to add the
demosaic USE flag.
You can do it editing the:
/etc/portage/package.usefile, adding an entry for that package like this:
- Code: Select all
media-libs/libraw demosaic
(you have to be root to edit that file)
or you can install it like this:
(as root)
- Code: Select all
USE="demosaic" emerge -1av libraw
(the first method is the recommended one)
3) now install the desired version of
darktable:
(as root)
- Code: Select all
emerge -1av =media-gfx/darktable-0.9.1