_express.js

var mongoose   = require( 'mongoose' )
  , ObjectId = mongoose.Schema.Types.ObjectId
  , Board      = mongoose.model( 'Board' )
  , BoardAdmin = mongoose.model( 'BoardAdmin' )
  , Post       = mongoose.model( 'Post' )

exports.show = function(req, res) {

    var data = {}

    Board
      .findOne({name: req.params.board})
      .exec()
      .then(function(board) {
        data.board = board
        return BoardAdmin.findOne({_board: board._id, _user:req.user._id}).exec()
      })
      .then(function(boardAdmin) {
        return Post
          .find( {_board: boardAdmin._name} )
          .where('_id').nin(boardAdmin.hidden_posts)
          .exec()
      })
      .then(function(posts) {
        data.post = posts
        res.render('content/boards/show', data)
      })
      .then(null, function(err) {
        console.log(err)
      })
      .end()
}

board.js

var boardSchema = new Schema({
    name       : { type: String, required: true, trim: true },
    title      : { type: String, required: true, trim: true },
    post_count : { type: Number, default: 0 },
    updated_at : { type: Date,   default: new Date() },
    master     : { type: ObjectId }
});

board_admin.js

var boardAdminSchema = new Schema({
    _user         : { type: ObjectId ,                 ref: 'User' },
    _board        : { type: ObjectId , required: true, ref: 'Board' },
    stared_posts  : [ { type: ObjectId, ref: 'Post' } ],
    hidden_posts  : [ { type: ObjectId, ref: 'Post' } ],
    blocked_users : [ { type: ObjectId, ref: 'Post' } ],

});

post.js

var postSchema = new Schema({
    title     : { type: String   , required: true, trim: true  },
    content   : { type: String   , required: true, trim: true  },
    status    : { type: String   ,                 trim: true  },
    embed     : { type: Object                                 },
    comments  : { type: Array,  },
    _board    : { type: String   , required: true, ref: 'User' },
    _user     : { type: ObjectId , required: true, ref: 'User' },
    _replyTo  : { type: ObjectId ,                 ref: 'Post' },
    _crossForm: { type: ObjectId ,                 ref: 'Post' },
    updated_at: { type: Date,  default: new Date() }
});

I think u can merge two then calls into one:

      .then(function(posts) {
        data.post = posts
        res.render('content/boards/show', data)
      }, function(err) {
        console.log(err)
      })