In the new interface of the blogger you can get your own custom robots.txt in blogger.
Try to upgrade your blog to the new interface.
After that go to the admin dashboard
==> then choose Setting
==> then choose Search preferences
==> then choose Crawlers and indexing
==> then Custom robots.txt
==> Choose edit
==> then Enable custom robots.txt content?
==> click yes
==> then enjoy you custom robots.txt in blogger as you need.
Now I finish creating custom robots.txt in bloggerIf you need any help do not hesitate to contact us.