Michael,
Yes they are Blue agaves, I had two in pots that were over 3 ft. tall and wide. Keeping in pots up north is something hard to do. It got so bothersome to bring them in for overwintering here in zone 6. I used to trim the leaf tips off with finger nail clippers. Only the very tiny tip. Almost couldn't notice it, but didn't cut into you.
Gordon,
I also have Stapelia gigantea which blooms only this color. The red color type is Stapelia Grandiflora. I plan to purchase one this spring. Soil ph will have nothing to do with the flower color.
Although they will hybridized freely with other kinds of their group. You could end up with all different types, but will not change flower color from season to season form the same plant.
Hetty,
When I approach a big cactus like you plan on. First get plenty of help. Then use welder's gloves and a denim jacket completely buttoned up. Next, Good Luck.
Stush
