Maybe it was a temporary issue with their certificate or just an issue on your side because it's fine here (secure and their SSL certificate is valid/working).
Yes it was an expired SSL certificate, the autorenew with Lets Encrypt did not work.
Well, you should have your PGP signed "Bitcoin Blender Guarantee" to prove you used their website, right?
Otherwise, how would anyone know (or how would you even be sure) if you really used their website and not a phishing version of it?
In this case, for registered accounts there is a "Proof of Ownership" on the deposit page, a PGP signed message which purpose is so users can validate it and be sure the deposit addresses in the message was generated by me and they are not on a phishing site.
The "Bitcoin Blender Guarantee" you talk about is given on the Quickmix page, it contains both deposit address and destination addresses, so it is a much better proof to have in case I don't deliver the coins to its destination.