In it, she describes the countdown to her wedding on October 11th, and posts selfies of herself trying on dresses, sampling wedding cakes, trying out make-up and visiting a church in Oslo.
Hundreds of worried readers in Norway commented on the blog, and asked Thea if she was really going to marry a man 25 years her senior.
Then the hashtag #stoppbryllupet (stop the wedding) started trending on Twitter earlier this week.
But it has since emerged that the girl in the photos is actually 12-year-old Swedish actress Maja Bergström, who was recruited by the Norwegian arm of the global children’s charity Plan International.
The organization is aiming to raise awareness of the fact that 39,000 children around the world are forced into marriage every day.
The Local scored the first interview in English with her mother Helena Bergström, 34, who lives with Maja in Stockholm suburb Järfälla.
"I couldn't even look at her, I was so sad," she said, describing the moment 'Thea' tried on her dress for the big day and took selfies in the store.
"It was horrible to see her like that. My heart was broken when I saw my little girl in this big wedding dress," she added.
"But I was so happy that I didn't really have to give her away like other mums do. I was relieved that this was not my reality."
One of Thea's blog posts describes her realization that she may soon need to have a baby. She writes about her mother telling her off, adding: "She was pissed off and said I had to stop acting like a child, now that I will soon have my own family to take care of."
Maja's mother Helena says that in reality she is "incredibly proud" of her daughter, who she says is reconsidering her acting career and now wants to work with health awareness organizations when she grows up.
Thea writes about making place names for the wedding dinner. Photo: Plan International
Plan International says it recruited a Swedish girl to take part in the campaign, rather than a Norwegian one to avoid its secret leaking.
"Yes, the girl is Swedish. The aim of using a blonde girl in a campaign like this is to raise awareness about the girls that are married off against their will every day," Sofia Klemming Nordenskiöld from the Swedish arm of Plan International told The Local.
"It's a fact that when we as an organization talk about 12-year-old Nargis who was married last week in Bangladesh it's hard to make people react, but when you read about Scandinavian Thea marrying a 37-year-old man the reaction is totally different".
She added: "It's just how it works, like it or not. The important thing for us is that the issue is raised and Plan has got a huge spread through this campaign".
12-year-old 'Thea' and her fiancé Geir. Photo: Plan International
The schoolgirl's mother told The Local:
"We talked about it a lot after we got the call from a casting agent, but Maja was very happy to say yes right away. She wanted to help other children."
Helena Bergström added that her daughter was "very nervous" ahead of the fake wedding ceremony, which is taking place in Oslo on Saturday.
'Thea' will walk down the aisle alone, with an audience made up of real family members, actors, politicians and human rights campaigners.
Her mother said:
"Although it is a bit scary for her because there will be a lot of people there she is very glad to be the face of this campaign and hopes what she is doing can make a difference".