They are sent by BearShare clients. Some Gnutella clients, especially those that do not support GGEP extensions, will drop empty queries. If one is looking for a file based on the hash ( located in a GGEP extension) and not a keyword, the query might be dropped (since there's no keyword specified). So instead, the query contains at least one character (the "\") which prevents the query from being dropped. A "\" could therefore be considered a "blank" query.