Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_20 (population INTEGER, colour VARCHAR, construction_commenced VARCHAR)

### Question ###
What is the lowest population for the neighborhood with the color blue that commenced its construction before 1987?

### Accurate SQL ###

SELECT MIN(population) FROM table_name_20 WHERE colour = "blue" AND construction_commenced < 1987