Example solution for stackoverflow question: http://stackoverflow.com/q/39986844/3442851

CustomDeserializer.java

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class CustomDeserializer {

    public void doStuff() {
        try {
            String json = "{\"key1\":\"val1\",\"key2\":\"blank\"}";

            // item
            ObjectMapper mapperItem = new ObjectMapper();
            SimpleModule moduleItem = new SimpleModule();
            moduleItem.addDeserializer(Item.class, new ItemDeserializer());
            mapperItem.registerModule(moduleItem);
            Item item = mapperItem.readValue(json, Item.class);
            // map
            ObjectMapper mapperMap = new ObjectMapper();
            SimpleModule moduleMap = new SimpleModule();
            moduleMap.addDeserializer(Map.class, new MapDeserializer());
            mapperMap.registerModule(moduleMap);
            Map map = mapperMap.readValue(json, Map.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    class Item {
        String key1;
        String key2;

        Item(String key1, String key2) {
            this.key1 = key1;
            this.key2 = key2;
        }
    }

    public class ItemDeserializer extends StdDeserializer<Item> {

        public ItemDeserializer() {
            this(null);
        }

        public ItemDeserializer(Class<?> vc) {
            super(vc);
        }

        @Override
        public Item deserialize(JsonParser jp, DeserializationContext context) 
                throws IOException {
            JsonNode node = jp.getCodec().readTree(jp);
            String key1 = node.get("key1").asText();
            String key2 = node.get("key2").asText();
            if (key1.equals("blank")) {
                key1 = null;
            }
            if (key2.equals("blank")) {
                key2 = null;
            }
            return new Item(key1, key2);
        }
    }

    public class MapDeserializer extends StdDeserializer<Map<String, String>> {

        public MapDeserializer() {
            this(null);
        }

        public MapDeserializer(Class<?> vc) {
            super(vc);
        }

        @Override
        public Map<String, String> deserialize(JsonParser jp, DeserializationContext context) 
                throws IOException {
            // definitely not the best way but it works...
            Map<String, String> map = new HashMap<>();
            String[] keys = new String[] {"key1", "key2"};

            JsonNode node = jp.getCodec().readTree(jp);
            String value;
            for (String key : keys) {
                value = node.get(key).asText();
                if (value.equals("blank")) {
                    value = null;
                }
                map.put(key, value);
            }
            return map;
        }
    }
}