I have read the big stopper has a bit of a blue cast. Been thinking about giving singh-ray a try. http://www.singh-ray.com
They seem expensive but well regarded.
I think most, if not all of the really dark ND filters have a slight blue cast, but it does give a distinctive look that doesn't actually look unnatural. I still have my Singh-Ray reverse grad filter for the Cokin set and it is the one filter I miss the most since upgrading to Lee. I want to get the larger size to fit the Lee set eventually, but the exchange rate isn't good at the moment. While Hi-Tech are good for the price (and I still have those too), they aren't in the same league as Lee and Singh-Ray.
You are correct about the blue cast of really dark ND filter. The good thing about the Lee, Big Stopper for instance, the color cast is even throughout the frame. I've had the singh-ray which had uneven color cast.
I tried to save money when I got into photography and got a set of solid and a set of graduated Hitech filters. What a waste of money that was. Then someone recommended Singh-ray filters. Did my research this time but the actual product was very disappointing. Twice the price of the Hitech but the quality is similar. Then I finally purchased Lee filters from 2filter.com
From my experience, I would have saved money if I had purchased Lee in the first place. The US stocks are very good now and 2filter usually ships within 2 weeks. But if you're looking at budget filter, I would recommend HiTech and just accept the fact that you will have more work in post processing. I would also say that avoid Singh-ray if you can. They are just as bad as Cokin but with better marketing gimmick (i.e., branding with famous photographers' name).