What do you want `blame --porcelain` to do that it doesn't? Using:

    git blame --line-porcelain "$1" -L "$2,"$2" |
    perl -MPOSIX=strftime -lne '/^author-time (\d+)/ and print strftime("%Y", localtime($1))'

I suppose it would be a little more convenient if you could ask `git blame` to format the whole line itself, but that wouldn't be part of the `--porcelain` output.

All that said, that pipeline is quite slow on something like linux.git, as it runs a series of blames which will walk over the same history many times. I think:

    git log -STODO --format=%ad --date=short

would be much faster (it's not _quite_ the same thing, as it counts TODOs which went away, but is a reasonable variant).