This file is indexed.

/usr/lib/mysql-testsuite/r/blackhole.result is in mysql-testsuite-5.6 5.6.16-1~exp1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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#
# Bug #11880012: INDEX_SUBQUERY, BLACKHOLE,
#                HANG IN PREPARING WITH 100% CPU USAGE
#
CREATE TABLE t1(a INT NOT NULL);
INSERT INTO t1 VALUES (1), (2), (3);
CREATE TABLE t2 (a INT UNSIGNED, b INT, UNIQUE KEY (a, b)) ENGINE=BLACKHOLE;
SELECT 1 FROM t1 WHERE a = ANY (SELECT a FROM t2);
1
DROP TABLE t1, t2;
End of 5.5 tests 
#
# Bug#13948247 DIVISION BY 0 IN GET_BEST_DISJUNCT_QUICK WITH FORCE INDEX GROUP BY
#
CREATE TABLE t1(a INT, b INT, c INT, KEY(c), UNIQUE(a)) ENGINE = BLACKHOLE;
SELECT 0 FROM t1 FORCE INDEX FOR GROUP BY(a) WHERE a = 0 OR b = 0 AND c = 0;
0
DROP TABLE t1;
End of 5.6 tests