The technical block "end portal" has the purple/black "unknown" texture in the hotbar and is invisible when holding the item.
The name is tile.null.name
/give @p end_portal
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discovered while testing
is duplicated by
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Confirmed
It is because, it is not a block in the creative menu, meaning that unless you are a map creator you shouldn't probably be using it, or need to use it. It is the same with the nether portal blocks.

@Joshua Merrett: Thought that too, but may be a bug.
Kumasasa, I found another bug with this block, I will report it now.
Even better when ya break End Portal, the portal parts itself, u get the water & lava particles lol

@Warren Liddell that is MC-48089
Still in 07a. However, the Nether Portal block has a texture and a name.
Confirmed for 08a
confirmed for 10b/c
Marking Works as Intended per Mog's response on MC-50972
I'm not sure that's intended...