Painfully slow (and crudely implemented) but a combination of svn log and svn cat works if you are searching the history of single files or small repositories:
svn log filetosearch |
grep '^r' |
cut -f1 -d' ' |
xargs -i bash -c "echo '{}'; svn cat filetosearch -'{}'"
will output each revision number where file changed and the file. You could always cat each revision into a different file and then grep for changes.
PS. Massive upvotes to anyone that shows me how to do this properly!