Dan Carter in a Blues jersey might be hard to swallow for Crusaders fans but it's reality after the franchise confirmed his signing today ahead of the Super Rugby Aotearoa season.

Carter trained alongside fellow All Blacks number 10 Beauden Barrett as he wore blue for the first time.

The Blues released a video on social media of Carter in his Blues strip before an expected confirmation after the training session. Carter played 141 games for the Crusaders, winning three titles, before shifting north to Japan after calling time on his illustrious test career following the 2015 World Cup.

"Two things from the lockdown that I realised was that I really enjoyed spending more time with my family and that I miss rugby," Carter said in a release.

"Leon is a good mate and we spoke about me helping out. For me it is a chance to mentor some young players and to give back to New Zealand Rugby.

Carter said he won't be ready to play next week when the Blues open their competition against the Hurricanes at Eden Park.

"I have not played for several months so it will be a number of weeks before I will be ready to be considered to play. And then only if form warrants it."

The Blues are thought to have been in talks with Carter for some time and a foot injury to Stephen Perofeta, who has been playing fullback for the side, in training this week may have fast-tracked the chance to sign Carter on a replacement contract.

Carter is a free agent after finishing his playing commitments with Japanese club Kobe Steelers when the season there was cancelled due to the coronavirus pandemic, and he has since returned to Auckland with his family.

Thirty-eight-year-old Carter was in fine touch in Japan during the past three seasons and his motives for potentially joining the Blues are said to be giving back to New Zealand rugby by sharing his knowledge and experience in the swansong of his career.

Signing Carter, the dual World Cup winner and three-time world player of the year, as a replacement player would also only cost the Blues $1800 per week.

The Blues are expected to hand star recruit Barrett the playmaking reins for their opening match of the Super Rugby Aotearoa competition against the Hurricanes at Eden Park but they also have fellow first five-eighth Otere Black, who was a major factor in steering the team to five wins from seven games before the season shutdown.

Carter's surprise presence would be the icing on the playmaking cake, giving the Blues depth beyond their wildest dreams, especially in a New Zealand derby competition that promises to be brutally combative.

Speaking to Irish first-five Johnny Sexton recently, Carter suggested players could now push their careers into their 40s.

"Using this time off, it's something you don't get as a professional rugby player," Carter said. "Having two or three months, maybe it's going to be much longer, of not having that constant grind, that contact. So it is like a mini sabbatical. I was very fortunate to have a couple through my career, but if it's used wisely the young players that have been playing heavily for the past four or five seasons, it's perfect timing.

"Then you look at the other side of the spectrum with more experienced players like yourself [Sexton], you don't get many opportunities like this. So, if it's used wisely and you keep training, and as long as your motivation upstairs is still there, I can't see why you can't play longer than potentially you thought you might, with having a break like this.

"Obviously, there is some pretty exciting rugby around the corner over the next couple of years, so I told Maro [Itoje, last week's guest] that I expect him to still be playing when he's 38 years old, like myself, and with you having a break like this then you can probably push through to the 40s. So, I'm looking forward to seeing that."

Carter potentially joining the Blues would leave New Zealand rugby stacked with world-class playmakers – a lineup that includes Barrett, Richie Mo'unga at the Crusaders and Aaron Cruden, who was in superb form with the Chiefs after returning from France.

The Blues did not respond to media requests for comment.