Defining a field as boost removes this field's properties. Thus, it is impossible to sort on the boost field. The last search with sorting the bothdocs shows the problem with "sort" : [ "-Infinity" ].

boostsorttest-verbose.sh

curl -XDELETE 'http://localhost:9200/boosttest'
curl -XPOST 'http://localhost:9200/boosttest' -d '{
	"mappings" : {
		"sortdoc": {
			"properties" : {
				"my_boost":{
					"type":"float"
				}
			}
		},
		"boostdoc": {
			"_boost" : {
				"name" : "my_boost",
				"null_value" : 1.0
			}
		},
		"bothdoc": {
			"_boost" : {
				"name" : "my_boost",
				"null_value" : 1.0
			},
			"properties" : {
				"my_boost":{
					"type":"float"
				}
			}
		}
	}
}'

curl -XGET 'http://localhost:9200/boosttest/sortdoc/_mapping?pretty=true'
curl -XGET 'http://localhost:9200/boosttest/boostdoc/_mapping?pretty=true'
curl -XGET 'http://localhost:9200/boosttest/bothdoc/_mapping?pretty=true'

curl -XPUT 'http://localhost:9200/boosttest/sortdoc/1' -d '{ "text": "test tast tost", "my_boost": 15.0 }'
curl -XPUT 'http://localhost:9200/boosttest/sortdoc/2' -d '{ "text": "test tast tost", "my_boost": 5.0 }'
curl -XPUT 'http://localhost:9200/boosttest/sortdoc/3' -d '{ "text": "test tast tost", "my_boost": 25.0 }'
curl -XPUT 'http://localhost:9200/boosttest/sortdoc/4' -d '{ "text": "test tast tost", "my_boost": 100.0 }'
curl -XPUT 'http://localhost:9200/boosttest/sortdoc/5' -d '{ "text": "test tast tost", "my_boost": 20.0 }'

curl -XPUT 'http://localhost:9200/boosttest/boostdoc/1' -d '{ "text": "test tast tost", "my_boost": 15.0 }'
curl -XPUT 'http://localhost:9200/boosttest/boostdoc/2' -d '{ "text": "test tast tost", "my_boost": 5.0 }'
curl -XPUT 'http://localhost:9200/boosttest/boostdoc/3' -d '{ "text": "test tast tost", "my_boost": 25.0 }'
curl -XPUT 'http://localhost:9200/boosttest/boostdoc/4' -d '{ "text": "test tast tost", "my_boost": 100.0 }'
curl -XPUT 'http://localhost:9200/boosttest/boostdoc/5' -d '{ "text": "test tast tost", "my_boost": 20.0 }'

curl -XPUT 'http://localhost:9200/boosttest/bothdoc/1' -d '{ "text": "test tast tost", "my_boost": 15.0 }'
curl -XPUT 'http://localhost:9200/boosttest/bothdoc/2' -d '{ "text": "test tast tost", "my_boost": 5.0 }'
curl -XPUT 'http://localhost:9200/boosttest/bothdoc/3' -d '{ "text": "test tast tost", "my_boost": 25.0 }'
curl -XPUT 'http://localhost:9200/boosttest/bothdoc/4' -d '{ "text": "test tast tost", "my_boost": 100.0 }'
curl -XPUT 'http://localhost:9200/boosttest/bothdoc/5' -d '{ "text": "test tast tost", "my_boost": 20.0 }'

curl -XPOST 'http://localhost:9200/boosttest/_refresh'

curl -XPOST 'http://localhost:9200/boosttest/sortdoc/_search?pretty=true' -d '{
	"query": {
		"match_all": {}
	},
	"sort": [
		{
			"my_boost": {
				"order": "desc"
			}
		}
	]
}'

curl -XPOST 'http://localhost:9200/boosttest/boostdoc/_search?pretty=true' -d '{
	"query": {
		"term": {
			"text": {
				"value": "test"
			}
		}
	}
}'

curl -XPOST 'http://localhost:9200/boosttest/bothdoc/_search?pretty=true' -d '{
	"query": {
		"term": {
			"text": {
				"value": "test"
			}
		}
	}
}'

curl -XPOST 'http://localhost:9200/boosttest/bothdoc/_search?pretty=true' -d '{
	"query": {
		"match_all": {}
	},
	"sort": [
		{
			"my_boost": {
				"order": "desc"
			}
		}
	]
}'