To add a target like _blank

1- Go to administration inside text format (admin/config/content/formats)
and choose the format you want to allow target.

2- Activate the filter HTML Purifier (advanced)

3- at bottom of option there is a tab to configure html purifier, the second option is AllowedFrameTargets
in that field add the target you want to allow in this case: _blank (check the picture attachment).

4- go bottom of the page and save change.

5- Delete cache