Just shows a fullscreen white color, press ESC to exit, I use it to wipe my screen :P

blank.c

#pragma comment(lib, "user32.lib")
#pragma comment(lib, "gdi32.lib")
#include <Windows.h>

LRESULT CALLBACK WndProc(
    HWND   hWnd,
    UINT   Msg,
    WPARAM wParam,
    LPARAM lParam
)
{
    switch (Msg) {
    case WM_KEYDOWN:
        if (wParam == VK_ESCAPE) {
            PostQuitMessage(0);
        }
        return 0;
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }

    return DefWindowProcA(hWnd, Msg, wParam, lParam);
}


int APIENTRY WinMain(
    HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPSTR     lpCmdLine,
    int       nCmdShow
)
{
    WNDCLASSA WindowClass = { 0 };
    WindowClass.lpszClassName = "Blank";
    WindowClass.lpfnWndProc = WndProc;
    WindowClass.hbrBackground = CreateSolidBrush(RGB(255, 255, 255));

    RegisterClassA(&WindowClass);

    HWND Window = CreateWindowA(
        WindowClass.lpszClassName,
        "Blank",
        WS_POPUP | WS_VISIBLE,
        0, 0,
        GetSystemMetrics(SM_CXSCREEN),
        GetSystemMetrics(SM_CYSCREEN),
        NULL,
        NULL,
        hInstance,
        NULL
    ); 

    ShowWindow(Window, nCmdShow);

    MSG Msg = { 0 };
    while (GetMessageA(&Msg, NULL, 0, 0)) {
        TranslateMessage(&Msg); 
        DispatchMessage(&Msg);
    }

    return 0;
}