The heart Unicode's own FAQ reaches for when it needs an example

Cool where the red heart is warm, the blue heart tends to get sent when the feeling is steady rather than fiery: loyalty, trust, calm, the kind of affection that doesn't need to announce itself. Blue is the colour we attach to reliability and depth — still water, clear sky — and the heart borrows all of that.

It has also picked up a few adopted meanings. For years it has been used each April to mark autism awareness, riding on the "Light It Up Blue" campaign that turns buildings and profile pictures blue on World Autism Awareness Day; you'll see clusters of blue hearts around that date carrying no romantic sense at all. Sports fans, too, use it as a team colour more than a declaration of love.

That flexibility is the practical thing to keep in mind. Because blue is so tied to causes, teams, and platonic loyalty, it's a poor choice for signalling romance — send a red heart if that's the message. The blue heart is better understood as the "I've got your back" heart: dependable, unshowy, and easy to send to a friend without the gesture being misread as something more.