SELECT
	testtable1.id,
		CASE
			WHEN (testtable1.tn_id IN
		SELECT
			b."customer_id"
		FROM
			cu.customer b)
		 THEN 'Yes'
		ELSE 'No'
	END) BLACKLIST
FROM
	testschema.testtable1

SELECT testtable1.id,
       CASE
          WHEN testtable1.tn_id IN (SELECT b."customer_id" FROM cu.customer b)
          THEN 'Yes'
          ELSE 'No'
       END blacklist
  FROM testschema.testtable1

SELECT testtable1.id,
   CASE
      WHEN exists (SELECT 1 FROM cu.customer b
        where b.customer_id=testtable1.tn_id)
      THEN 'Yes'
      ELSE 'No'
   END blacklist
FROM testschema.testtable1

WITH
	tbl AS
		(	Select 'A1' "ID", 1 "TN_ID", 'C' "STATUS" From Dual Union All
			Select 'A2' "ID", 2 "TN_ID", 'A' "STATUS" From Dual Union All
			Select 'A3' "ID", 3 "TN_ID", 'B' "STATUS" From Dual Union All
			Select 'A4' "ID", 4 "TN_ID", 'B' "STATUS" From Dual Union All
			Select 'A5' "ID", 5 "TN_ID", 'A' "STATUS" From Dual 
		),
	cust AS
		(	Select 1 "CUSTOMER_ID", 'Name 1' "CUSTOMER_NAME" From Dual Union All
			Select 3 "CUSTOMER_ID", 'Name 3' "CUSTOMER_NAME" From Dual Union All
			Select 4 "CUSTOMER_ID", 'Name 4' "CUSTOMER_NAME" From Dual 
		)

SELECT	t.id,
        CASE  	WHEN t.tn_id IN( SELECT CUSTOMER_ID FROM cust ) THEN 'Yes'
        ELSE 'No'
        END "BLACKLIST"
FROM	tbl t

SELECT	  t.ID, CASE Nvl(c.CUSTOMER_ID, 0) WHEN 0 THEN 'No' ELSE 'Yes' END "BLACKLIST"
FROM	  tbl t
LEFT JOIN cust c ON(c.CUSTOMER_ID = t.TN_ID)
ORDER BY  t.ID

SELECT	  t.ID, CASE WHEN EXISTS(SELECT CUSTOMER_ID FROM cust WHERE CUSTOMER_ID = t.TN_ID) THEN 'Yes' ELSE 'No' END "BLACKLIST"
FROM	  tbl t
ORDER BY  t.ID 

ID BLACKLIST
-- ---------
A1 Yes       
A2 No        
A3 Yes       
A4 Yes       
A5 No