According to the Theoben wesbsite, they installed their own in-house bluing tanks & polishing gear in May of '96.
Venom definitely did used to do their blueing for them at some point before this.
HTH. <img src="http://jsramsbottom.co.uk/bbs/e/icon_wink.gif" alt="Wink" width="15" height="15"><!--graemlin:-->