How many blocks should I use in the gradient?
From 3 to 12. With few blocks the step is obvious; with many, the difference between neighbors shrinks, but every extra block is one more material to gather. For a tall wall, 5 to 7 is usually a good balance.
Can I pick any block as an end?
Yes: the picker accepts all 336 blocks, even those the filters hide. The blocks in the middle, however, only come from the families turned on in the filters, which by default leave out ores, functional blocks and live coral.
Why does the same block not appear twice?
Each step excludes already chosen blocks, so the gradient does not repeat a block when two neighboring points land near the same one. If candidates run out, the step is left empty instead of repeating one.