Nice. I suspect that you have insufficiently-powerful colour-developer then, or the pH is wrong. When doing E6, weak shadows are an indication that you should run the CD longer while colour shifts (usually blue/yellow but it depends on the film brand) can be adjusted by changing the pH of the CD mixture. If I were you, I'd try it in C41 or E6-CD for the colour dev stage.
However, slides are known to colour-shift with age and I think it's at least partially due to degradation of the dye couplers. It's possible that some of the couplers in the film are so far gone at this point that you just won't get a real black.
I'd love to see some scans.