1. A computer-implemented method for searching a content collection, comprising: under control of one or more computer systems configured with executable instructions, receiving a search request specifying query content to be matched with content pieces in the content collection, each content piece characterized by content descriptors, at least a subset of the content descriptors for each content piece corresponding to one or more blur transforms; identifying query descriptors for the query content, the query descriptors characterizing the query content; for each content piece: identifying the subset of the content descriptors of the content piece that corresponds to at least a portion of the query descriptors; identifying first candidate regions of the content piece, one or more of the first candidate regions overlapping with at least another one of the first candidate regions, each first candidate region corresponding to at least a portion of the subset of the content descriptors; identifying second candidate regions of the query content, each second candidate region corresponding to at least a portion of the subset of the content descriptors; and selecting the content piece for inclusion in a matching content subset of the content collection when at least one of the first candidate regions comprises a first proportionate size greater than a first proportion threshold or at least one of the second candidate regions comprises a second proportionate size greater than a second proportion threshold; and providing for presentation at least reference to the matching content subset in response to the search request.