Help with building Chrom

Honestly, his default kit works just fine, just replace his C skill with something more beneficial to the team.

As far as blessings, those don't really matter unless you're putting him on a team with Fjorm, Gunnthra, Ephraim, or Ike. With that said, most of the time, Blessings are usually used for the blessing game mode (can't remember the name, it's the one where you can only use a team full of units with a certain blessing), extra SP, and quests. They're also used for themed teams (Wind Blessing-only team, etc), but those are preferences.

So, basically, blessings aren't really that important unless you're putting them on a team with a specified unit, and even then it's not all that important.

A neutral Chrom can work really well with his weapon. I’m not too familiar with his base set, but defidently keep fury and chill defense. The sheriff skill is up to you, or corse everyone likes reposition, but since Chrom is kind of tanky swap can work too (depends on your teams and other units). As for blessings, unless you plan to use him on a team it won’t really matter. I usually just make sure that within each blessing group that I have a couple people of each color and general type (hyper offense, mixed tank, flier, you catch the drift).

Mine is +HP -Res and I'm running him with S.Falchion, Swap, Aether, Fury 3, Chill Def 3, and Hone Cavalry. I know it's not optimal but he performs well enough for me!
I've also got an Earth Blessing on him but I haven't used him much in blessing-only teams.