Good blue merge project

Nowi is the prime example of a great tanky blue unit, her Def and Res and Spd are balanced. She automatically comes with lightning Breath which has built in DC. The only drawback is that the required skill set is a bit expensive.

Effie is the definition of a heavy hitter. With damage outputs of over 55 Atk, she can OHKO most red foes. Her skill set also is expensive, but there are some budget builds.

Shigure is the fastest blue flier in the game with 37 Spd at neutral. He also brings a new ( at that time ) Woa Do lance which is amazing for builds with Wrath. He can even utilise Firesweep lance and LnD set.

The infamous Reinhardt is one of the best units in the entire game. With having his default brave tome paired with death blow, Reinhardt can cause destruction to all red units. He is everyone's worst nightmare.

IMO, Those are some of the Blue units that are considered good to have as merge projects, there are other possibilities, it's just how you build the units.

Any 3-4* is perfectly fine, you just have to choose the one you like the statline/character of the most. If you're unsure you can always look at the arena tier list to see which characters are more valuable overall. Here are some of my recommendations:
Lukas - Amazing Def, physical wall (I think the best non-armour def?)
Effie - Great armoured unit, highest attack in the game (Gwendolyn is also good if you want a more balanced armoured unit).
Cordelia - One of the best offensive statlines for a lance flier and the most accessible out of the three 'best' (Cordelia 35/35, Tana 34/36 & Shigure 33/37).
Oscar - Best lance cav excluding L!Ephraim and S!Cordelia (Camus is good too - similar stats + distant counter wep, but he's a ghb so limited merges and no boon/bane).

Both Nowi and F!Corrin are amazing due to being dragons, although Nowi typically outclasses F!Corrin in everything except speed.

As for mages Reinhardt is still one of the best heroes in the game, after that I guess Tailtiu? The best non-5* bladetome mage (and its in her base kit) due to her out speeding all other 3-4* blue mages, keep in mind she comes with Drive Spd however so you'll lose out on that skill until you finish merging.

Aside from that, I know they are 5* exclusive but L!Ephraim and Fjorm are great long-term projects because blessings grant higher arena scores.

Sorry for the long post but I hope this gives you some idea as to a character to choose.

Nowi, Effie, Wendy, Shigure/Cordelia, F!Corrin, and Tiltyu are all good non-5* exclusive merge projects

*Sees a post asking about blue merge projects*
*Doesn't see anyone mention Donnel*

You guys are gonna trigger me so hard.

No, but in all seriousness, a lot of the 3* and 4* blue units are really good. Donnel, Lukas, Mae, Shigure, Catria, Clair, Est, Cordelia (even if I do hate her art...)...

Donnel is a very unique unit who can be offensive or defensive. You can invest into any stat as he can be really tanky. A SB/WB QR Donnel can be intimidating and your also going to have trouble with his high Hp.

If you like Hardin, I would say Nowi is the closest thing in the 3*/4* pool.

My favorite is hands down Catria (Still looking for that elusive +Res/-HP IV for her). Give her Berkut's Lance+ (refined) and cater to her defensive side and she will become a Flying Hardin!