PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT ?person ?blog
WHERE { 
   ?doc rdf:type foaf:Document; foaf:maker ?author .
   ?author foaf:knows ?person.
   OPTIONAL { ?person foaf:weblog ?blog. }
}