A really interesting 'Doctor X Tardis' fanfic I've come across is 'Tardis Tales'. In this fanfic, the Tardis has a sort of consciousness that the Doctor tries to fully understand. It delves into the mystery of the Tardis' inner workings and how it has a connection to the Doctor on a deeper level than just being a ship. It's a short but engaging read that will make you look at the Doctor - Tardis relationship in a new light.
Good character development in both the Doctor and the Tardis is key. If a fanfic delves into the Doctor's past experiences with the Tardis and how it has shaped him, and at the same time shows the Tardis evolving in some way, it's bound to be great. For instance, the Tardis could develop new functions based on the Doctor's needs over time. Also, a vivid description of the Tardis' interior and how it feels to be inside it can really make a fanfic stand out.
Well, they might end up in a completely unexpected time or place. Maybe a planet full of strange creatures that they've never encountered before. It could be a really wild adventure.
Often, it involves exploring the unique bond and adventures between the human Tardis and the Doctor. There's a focus on how this unconventional setup changes their dynamic and the challenges they face together.
Since I don't have first - hand knowledge of this fanfic, my best guess is that it could involve a situation or concept that is more 'blue' in nature compared to the TARDIS. Maybe it has to do with a different kind of blue - hued alien or a blue - tinted dimension that is explored in the story. It could also be a play on words, where 'bluer' refers to something other than the color, like a 'bluer' state of mind, and the story then weaves this concept in relation to the TARDIS, perhaps showing how this 'bluer' thing interacts with or is different from the TARDIS.