Well, the original versions on TV and home video were always too bright, which made the "blob" stand out much more. It would have been very difficult to see in a theatrical presentation in either 1983 or 1997, much like garbage mattes.

The 2004 DVDs, however, were too dark, making the blob even harder to see. Plus, on the one shot when it's clearly visible (the 'saber cross in front of the laughing Emperor), the lightsaber cores are all sorts of messed up.

Harmy, in fixing the lightsabers in this shot, had to brighten it up to the point where the blob is, unfortunately, pretty clearly visible once more.

But I'd rather have fixed lightsabers than fix the Emperor's head slug.