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&x2665; on a black background will render nothing. (black heart on black background)
Wouldn't it be 'nicer' to be replaced with
&x2661; (white heart) ?
Your premise is incorrect. ♥ U+2665 BLACK HEART SUIT is not actually black. Unicode character names use the word “BLACK” to refer to the “foreground” or “ink” color of the text, not the color black.
(Whether or not that was the intent of the Unicode specification authors, it is certainly how every font renderer I have ever met does things, though I admit with the recent exception of these newfangled “emoji” characters, which should get off my lawn.)
When there are “BLACK” and “WHITE” versions of a character, the “BLACK” refers to a solid version whereas the “WHITE” one is ♡ outlined. If you copy the text of this answer and apply a white-on-black style, the “BLACK” heart will be white and the “WHITE” one will be a white outline.