What is the correct title for this poster?

My son has an educational poster and the title bothered me. I thought it should be "Blending Consonants Is Fun", but couldn't fully explain to him why. It seemed to me that because it's clearly correct to say "blending is fun", rather than "blending are fun", then it should also be correct no matter what we are blending, whether or not that thing is plural.

Which is correct and why?

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"Blending consonants" is a noun phrase describing consonants that form a cluster. When parsed that way, "are" is correct (although "fun" might still be up for dispute).