some information from reddit, may help you.
"I've been looking for a solution as well but Lunar's Blackout just seems to be a waste of money IMHO. So are other apps provding such a feature.
So what I did was just make the MacBook's screen mirror one of the two external monitors. After that I just dimmed that Macbook screen and just enjoy using two external monitors. Of course I have have to reverse the dimming of the Macbook screen beore I disconnect my extenal monitors.
Hope that helps you OP.
On a side note, Windws as Windows Key + P to give you this arrangment. It blows my mind Apple does not have this natively."
https://www.reddit.com/r/mac/comments/1bwy57b/how_can_i_use_2_external_monitors_and_keep_lid/