Here is an example of how to use the SqlBlobSerializer class in the com.fasterxml.jackson.databind package to serialize a java.sql.Blob object into a JSON string:

import com.fasterxml.jackson.databind.ObjectMapper;
import java.sql.Blob;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class Example {
    public static void main(String[] args) throws Exception {
        // Connect to the database and retrieve a Blob object
        Connection conn = DriverManager.getConnection("jdbc:mysql://host:port/database", "username", "password");
        PreparedStatement stmt = conn.prepareStatement("SELECT data FROM files WHERE id = ?");
        stmt.setInt(1, 1);
        ResultSet rs = stmt.executeQuery();
        rs.next();
        Blob data = rs.getBlob("data");

        // Serialize the Blob object to a JSON string
        ObjectMapper mapper = new ObjectMapper();
        String json = mapper.writeValueAsString(data);

        // Use the JSON string as needed
        System.out.println(json);
    }
}

In this example, the SqlBlobSerializer class is used to serialize the Blob object into a JSON string. The writeValueAsString() method is used to convert the Blob object into a JSON string, which is then printed to the console.

It's important to notice that this is a basic example, and you may need to adapt it to your specific use case and database connection. Also, the SqlBlobSerializer is not a class that you need to instantiate, it is used automatically when trying to serialize a java.sql.Blob object.

Also, you should be aware that the JDBC is a low-level API that is not recommended to use in a high-performance and high-concurrency environment, it is preferable to use an ORM or a connection pool to handle the data access.