Here's a 1-KiB program that does that... no .NET Framework or anything else needed. :)
The source code is literally 3 lines:
#include <windows.h>
int main(void)
{ return SendMessage(GetShellWindow(), WM_SYSCOMMAND, SC_MONITORPOWER, 2) != 0;}
Base64 encoding of executable:
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