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.

Just as with zinnias, the first cutting is going to be below the first set of lateral branches. This is essentually "pinching." And, it is also going to force the plant to branch and produce nice long stems. In the field, we would space euphorbia marginata 12" apart. It's also one that we would do successive sowings two weeks apart. This is going to be our eight season selling cutflowers at market. We've never made a habit of counting stems.

One theory on the black spots... All plants were sprayed with Daconil out on the balcony. I left them to dry then brought them in. It's possible there were still some drops of it left on the plant. Could this possibly be from the sun hitting the plant and causing some burning? I sprayed them in a shady spot, but the window it's in is very sunny. Maybe I just put it back in the window too soon?

Afternoon sun. Planted in ground. San Clemente, California.