com sed
sed -n '
s,^,/something/,
1s/^/blahblahblah /
H
${g;s/\n/ /gp}
' file1.txt > file2.txt
com bash
( printf "blahblahblah"
mapfile -t lines < file1.txt
printf " /something/%s" "${lines[@]}"
echo
) > file2.txt
ou
{ printf "blahblahblah"
while IFS= read -r line; do printf " /something/%s" "$line"; done < file1.txt
echo
} > file2.txt