That's not quite the case. The blocks are available in the Miscellaneous Symbols and Arrows section of Unicode. That's usually rendered as plain text (although

That's not quite the case. The blocks are available in the Miscellaneous Symbols and Arrows section of Unicode. That's usually rendered as plain text (although not ASCII). But they're also paired with Variation Selector 16 (https://emojipedia.org/variation-selector-16/ https://emojipedia.org/variation-selector-16/) which gives them an Emoji presentation on most modern systems.