Impenetrable blue mask in cross-processing

As you all know, the "lomo" look and cross-processing E6 slide film in C-41 color negative chemistry has grown in popularity. I've been given, and also bought, some expired E6 film, mostly Ektachrome in 35mm and 4x5, and have been struggling to cross-process it. I know my C-41 chemicals (Rollei Digibase) are good, but most of the time all I get is an impenetrable, impossibly dark blue mask through which I can just barely discern an image. It's no good for scanning and I'd wager also no good for optical printing.

Are some films better than others for this treatment? Does the expiration have any bearing on this? Are there any ways to optimize my results, or should I just drop the whole idea entirely?

Thanks!

Try varying exposure. The blue is typical for a lot of my x-playing, but not so blue you can't see though.

John Callow has done more E6 in C41 than any person I know , maybe he will jump in here.

I've never had that problem, try taking those blue films and bleaching and fixing again.

How expired are these films? I'm not an expert on this, but I believe very old slide films had a dark layer on the back which needed exposing to light as part of the process. I think this was the E3 or E4 process, so i'm talking 30+ years, but it might be worth checking. The E6 process replicates this chemically and in my experience I've had some great results cross-processing. I don't know how these older slide films would work in C41.

Otherwise I've never had a colour cast from x-pro film that was so heavy it couldn't be scanned. They've needed a fair amount of tweaking during scanning to get the colours right, but nothing like what you have described.