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.

Most importantly is the plant inside against the wall all the time? This Euphorbia Ammak is a large growing tree and needs bright sunlight to maintain itself. Soil looks to be dry but they should be in a gritter mix to increase drainage. Is the brown spots mussy or soft, and/or smells funny. It is possible that it is still okay. Needs better living conditions. Turning brown or black is sometimes part of growing this white ammak. If you must cut, Use a sharp knife and cut across where there is no rot and only good stem. Have a spray bottle of cold water to spray the white latex sap. Sap is toxic and a skin irritant and wash right after cutting it. If you get any in your eyes, wash out with tons of water then go to the hospital for treatment. Let the cut heal and dry for a few days before planting in pure grit or perlite. It takes almost a year for new roots to form. No water can be taken up till it grows new roots. Google 'cutting my euphorbia ammak' for videos and more instructions.

"It looks nothing like other Senecio." I don't know which other Senecio s you're familiar with, but Senecio is a genius comprising many and varied forms from annual weeds, via herbaceous perennials and succulents, to shrubs. The two names are synonymous.