My Tumblr tag is a bit of a combination of my full username I use and my favourite colour. The backstory is a little dark.
I've always been joked by people around me and on the internet of not being human as I had always done a little too much- staying up at night, no one seeing me sleep ever in their lives as I was the last to sleep first to wake up... doesn't help I have iris that seem to have a red ring around it appearing not exactly human, so jokingly my roommates said "they are hunting humans at night that's why we never see them sleep or eat during the day" specifically they called me a ghoul like from Tokyo Ghoul since they were all otakus. It was funny, this wasn't an insult, just an inside joke.
I am pretty tough, I'm not afraid to stand up for myself or my friends so I tend to scare bad people away easily... way too easily. People don't expect you to defend yourself a lot of the time.
In some games I can be a crazy and be the one carrying the game. I'm not that good at all games I just happen to lock in whenever it's needed and be a monster. My friends often said they wouldn't want to be matched against me because even though I am clearly rusty because I open said game once a month I am somehow playing on a level way above mine. So I would often be nicknamed not human once again.
I am an agender on top of all of this, I don't care what pronouns people use for me, I use all them and I don't really care for myself in this way since I just want to be respected as a living being that's all. But this unusual stance towards my own pronouns adds another "cryptic" point to my name.
I also experienced and witnessed a lot of human death sadly, it's tied to me and my life to a point where I used to think I was cursed but nah, it's just life and my infamous misfortune.
Lastly here I'll get a bit more personal. I was in an abusive forced relationship for about two years. I used to have a different nickname on the internet but after I broke off and luckily escaped that relationship I changed everything, blocked that person everywhere I could so they couldn't stalk me. I rebranded to Cryptic Toofu, hoping they wouldn't find me. They still did so I failed... but hey I already had friends calling me by that name "Toofu" and it had a nice ring to it and I started to love it.
That part of my name has a less interesting story as I simply thought "what would be simple to remember but unique enough so it's not claimed anywhere by anyone" I used to have a problem of my old username being claimed often. I thought of Cryptic Tofu but then I added a second "o" to be like eyes, to be cute and more like a cryptic creature. The name is ironic considering I hardly eat tofu but I still like it, now I won't change it.
So the shorter answer is, cryptic is because of the multitude of jokes of me not being human and Cerulean is just another one of my favourite colours. My favourite colour is teal but "cryptic-teal" doesn't have a nice ring to it, sounds like a cryptic-seal.
And Toofu is just cute memorable branding. I never had a problem with anyone forgetting my name. Quite the opposite.
Anyways yeah that's the lore. If you ever see a different signature on my older paintings that's because my old username used to be a combination of my irl full name combined together and I used to use it for signature. I still sometimes use a very stylized version of it nowadays but it used to be more legible. Now there is no reason to make it legible...
I don't have anyone to tag, sorry.