alter table engine=blackhole takes lot of time

I decided to alter big myisam tables to blackhole and it took very long, like if it was converting to any other type

Example:
Query OK, 201147303 rows affected (8 min 9.60 sec)
Records: 201147303 Duplicates: 0 Warnings: 0

Actually I could make it faster by either dropping/recreating or truncating/alter

Maybe a small fix can be introduced? To make alter-to-blackhole happen faster regardless the original table size/engine?

Thanks!