Only looking for Elep. Ears that I don't have. Post here.

I've grown both and have seen them in a grower's setting as well. They ARE quite similar in appearance - in full sun, side by side, it's a bit difficult to tell them apart until flowering. 'Blackbird's bracts are not as intensely chartreuse or limey green as 'Purpurea'. They have more of a reddish tone to them and can be mottled with the same deep purple tones of the regular foliage, which in full sun can get very dark - nearly black. I've never seen this with 'Purpurea', which generally has very bright lime green/chartreuse bracts. 'Purpurea', being a straight cultivar of C. amygdaloides (wood spurge), is more shade tolerant as well. And a zone or two hardier. Not to be too nitpicky, 'Redwing' is a hybrid, being a cross of E. amygdaloides and x martinii. And obviously, E. x martinii is itself a hybrid, a naturally occurring cross of E. amygdaloides and characias. So there's a great deal of amygdaloides in 'Blackbird's parentage but enough characias to make it somewhat less hardy than 'Purpurea'. 'Purpurea' will self-sow under good conditions: 'Blackbird' is sterile and will not.

I don't have the answer. I have a Tropica book. The pictures in it seem to back your descriptions of the difference. The plant I have clones of I have called E. ingens. It came without a name from a neighbor. I have never thought it was a E. ammak. My I.D. confusion was between E. ingens and E. candelabrum. Here is a poor mood shot I have on my screen. The shots I took today will not appear as I failed to note there was no card in the camera. This has the growth spurt summer/winter affect you mention in an old "tall Euphorbia" article you penned. The fruit are concentrated at the growth tips. They stay yellow and succulent. The spine shields are mostly small. New growth from the bottom has some mild variegation that goes away. The trunk becomes grey/brown and tree like as it matures.

Does the soil take a month to dry? If it dries more quickly, I think your plants are thirsty.