Hi Neil,
I too have this filter and it works brilliantly. Although my dob is 2" larger than yours, it really brightens up the Orion Nebula. Perhaps you have a faulty filter or they gave you the wrong one? From what I can remember, it didn’t make this nebula look blue- it made it white.
Just for future reference, note that UHC and OIII filter work differently on different objects. Orion Nebula as an example is better seen through a UHC and planetary nebulas generally in an OIII. But regardless of which one, both would definitely have given you a much brighter image.
Here’s a very helpful link which you should find interesting regarding filters.
http://pages.sbcglobal.net/raycash/filters.htm