Thanx for the insights!
Fixed that, but was still having tool-change blobs (and was previously blobbing on all extruder changes, not just when engaging that one).
Played around with the various retraction settings, and it seems the culprit was the relatively large *difference* between the 'regular' and the 'tool change' retractions.
With the 6mm (regular) retraction and default 16mm 'nozzle switch retraction', you showed how Cura was first doing the E-16, Tx (tool change) then E+10 (=16-6, pre-priming?) of the new color. It's that E+10 on the new tool/color/extruder that causes the blob before moving to the prime tower, where it un-retracts 6mm before printing/priming on the tower. I set the 'nozzle switch retraction' to 6mm (matching the 'regular' retraction distance), and the blobs disappeared. This means I need more priming after the tool change to clear out the previous color, but increasing prime tower volume seems to help compensate, as well as printing walls from inside out to hide the still-transitioning color...
It's a mystery to me why Cura does the new tool un-retraction/pre-priming BEFORE moving over to the prime tower, but there it is. (probably just easier to program that way for the case where no prime tower is in use...)
Thanx again for the pointers! I think i can at least try a real multi-color print now with some hope of success...
PS, I think I grok the math of how it's trying to maintain the working nozzle pressure by pre-priming the new extruder by as much as it does, but perhaps the idle extruder sitting at temp leads to a very molten first squirt of the new extruder? ie, why else would not even equalizing the previous pressure cause a blob? (as you pointed out E-16, E+10 should leave the filament 6mm behind the tip, but it doesn't, for some reason) I may play around with a NEGATIVE value for "Nozzle Switch Extra Prime Amount" (instead of a giant positive one!) to see if that helps...