Her character's dishevelled brunette bob has been replaced by a peroxide crop, and there's not a baggy coat in sight - just a fitted floral shirt and navy skirt.
The new hairdo is in aid of a film - she'll be going back to her familiar style tomorrow ahead of filming Vera - but she's rather enjoyed being a blonde bombshell, remarking: "I have to say, you do get double looks. I quite like it."
On first meeting her, Blethyn seems quite the opposite of the straight-talking detective.
She's sweet, has a girlie voice and laughs a lot. The more she talks, though, the more it becomes evident that she's not all that different to Vera.
For one thing, they're both very down to earth. It's something that's often noted of actors, but with Blethyn, winner of multiple awards and holder of an OBE, it's quite startling.
Take her approach to accommodation while filming the show. Each series sees the cast heading to Northumberland for five months. Usually Blethyn stays in a hotel where she's waited on hand and foot, but this time the 67-year-old decided to rent a cottage.
"Sometimes I ask for a sandwich and all I want is two bits of bread, cheese and tomato. It comes up with three pieces of bread, crisps, the lot," she says, laughing.
"And the washing! I don't want a T-shirt to come back gift wrapped, I just want to go to the laundrette."
Blethyn and Vera are both happy spending time alone, too. During filming, she normally spends her evenings in solitude, learning the next day's lines. "I'm not lonely, and I don't think Vera is," she says.
Often her husband, art director Michael Mayhew, who she married in 2010 after 35 years together, will come up for the weekend.
"He can do his work up there. Now I've got a cottage he's got no excuse, actually."
While series three of Vera is about to hit our screens, Blethyn is about to begin filming the fourth.
"I couldn't believe they commissioned another one, I thought 'Cor, that's really nice'," she says.
So why has the show been such a hit? It focuses primarily on crimes, revealing little about the police squad's lives, there's no romantic liaison and Vera is far from glamorous. And those are just some of the reasons why Blethyn believes people enjoy it.
"And Vera isn't a threat to anybody. The husbands aren't thinking, 'Phwoar, I'd like to give her one!' Although I have had a few letters," she adds, chuckling.
For those who like a bit of eye candy with their murder mystery, there's Vera's sidekick, Detective Sergeant Joe Ashworth, played by Cutting It's David Leon. It must be nice working opposite that face every day?
"Oh, not half," says Blethyn. "I don't think he knows he's handsome. Did I tell you he's a film-maker? He's a very clever chap."
From The Fall to Broadchurch and Scott & Bailey, female TV detectives seem to be going through somewhat of a golden period right now.
"Somebody asked me if it's nice to see women come back. I said, 'What do you mean, come back? We've been here all along!'" exclaims Blethyn.
It certainly seems like she has, but Blethyn came to the game fairly late.
Having grown up as the youngest of nine in a working-class family in Kent, she left school at 17 and got a job as a shorthand typist for British Rail, where she met her first husband, Alan Blethyn. Acting was a spare time hobby.
After nine years, the Blethyns' marriage ended, but around the same time she took her acting up a notch, landing a place at the Guilford School of Acting.
Her career kicked off at the National Theatre and she made her TV debut in 1980, in Mike Leigh's Grown-Ups. Leading roles soon followed in a number of sitcoms, including Chance In A Million and The Labours Of Erica.
Her big screen debut was in The Witches in 1990, but it was Mike Leigh's Secrets & Lies, six years later, which made her a household name.
Since then, she's appeared in a range of films, from independent comedies like Saving Grace to big-budget dramas like Pride And Prejudice, and has been nominated for a string of big industry awards.
"I've been lucky," she says modestly. "But I'm not sure everyone would want to play the parts I do. Maybe they've been offered to everyone else before they get to me."
Blethyn's latest film, Enemy Way (for which she adopted the bleach blonde hair), sees her acting alongside Forest Whitaker, Harvey Keitel and other Hollywood celebs.
She does get star-struck, she admits, like the time Sidney Poitier walked into the party she was attending.
"I thought, 'Oh my God, it's Sidney Poitier', and he walked over and said, 'Hello Brenda, lovely to meet you'," she recalls, bursting into hysterics. "I couldn't believe it."
Despite that, she doesn't generally get too excited about the showbiz world. "I don't dislike it, but I dislike it if it's considered important, because it's not," she says. "We put [celebrities] on a pedestal."
For that reason, she's always happy to talk to fans when she's out and about.
"People come up to me and go, 'Oh, look at you doing your shopping, do you have bread as well?' I'm always polite, but there are days when I wish I'd combed me hair!"
So, when you've already achieved so much, where do you go next? Is there a genre Blethyn hasn't tried yet?
She pauses to think. "Melodrama," she finally offers. "Although it could be argued that everything I do is melodrama," she adds, laughing.
For now, though, there's Vera to film, and her new Nintendo Wii to keep her busy.
"I went to see my sister who's 80 and we spent four days playing on it," Blethyn says, rather excitedly. "You can do all sorts: skiing, tennis, running..."