I have never used a Lomo, but what shutter speed did you use? I don't believe this camera has a light meter, so how did you determine proper exposure?
I google'd the camera you mentioned, it doesn't look like it has a light meter, so if your film is black then you let too much light in.
If you keep the shutter open for a few seconds outside, it will be surely black.
What type of film did you shoot (b/w or color)?
What do the edges if the film look like, dark or clear'ish?