For passing output as an argument, I tend to use a while loop since I'm not familiar with xargs.
ls | grep '^Dar' | while read line; do rm "$line";done; Edit:
While still often using the above to process the output of one command into the argument of another command. I have familiarized myself somewhat with xargs:
Would ls be returning [a, b, c] then #> ls | xargs rm is equivalent to #> rm a b c. This might yield unexpected results due to possible spaces in the arguments.