I use unique email addresses for each site where I register an account.  Then if I start receiving spam to a site's email address I know that site has either sold me out to spammers or has been comprised.  I assume aurcade wouldn't sell my email address to spammers, so it's likely aurcade.com has been compromised.

The spam I received today was a blackmail email claiming they had a compromising video of me and they demanded $300 sent to a bitcoin address.

I'm not talking about the source address, the destination address (i.e. my email address) is unique for aurcade.  While you are correct there is an approximately a one in a bazillion chance is this a freak coincidence, for all practical reasons this means that at least aurcade.com's database of email address has been stolen by spammers.  I have no idea how aurcade.com protects its user info, so it's certainly possible/likely that more user info like usernames and passwords have also been compromised.

Sadly it's not uncommon for even big well funded websites/stores to have data breaches (Target, Yahoo, etc.), so at least aurcade.com is in good company. :)

For your own security you shouldn't reuse passwords on different websites.  The easiest way to do this is with a password manager.

If you've reused your email/password combo on any other websites I highly recommend you change your password on the other websites.