NR 2.1.2 states that
"The \skip command must be followed by a number,
but this number is ignored in lyrics which derive their durations
from the notes in an associated melody through \addlyrics or
\lyricsto. Each \skip skips a single note of any
value, irrespective of the value of the following number."
Which means that right now you have to do
<<
\new Voice = "foo" { b b b b b b b b }
\new Lyrics \lyricmode {
\skip 1 blah4 }
%% BUT:
\new Lyrics \lyricsto "foo" {
\repeat unfold 4 \skip 1 blah }
>>
I suggest that either \skip wouldn't require a number when in
automatically-aligned \lyricsto mode, or that the number indicates the
number of syllables to skip (rather than a duration), which would
allow users not to use the \repeat unfold hack.