/etc/blends/sources.list.unstable is in blends-dev 0.6.16.4ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | # using unstable as target distribution for the meta package dependencies
# does actually not sound reasonable. The idea is to enable a smooth transition
# to testing for all meta packages and thus here testing is used as target
# distribution. You are free to provide your own source.list.unstable
# in the source of your meta package building code to force unstable as
# target or alternatively you could change this file (/etc/blends/sources.list.unstable).
deb http://ftp.debian.org/debian testing main
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