Though I have used transparencies intended for overhead projectors, plain paper works fine for me if exposed to UV for a bit longer. If you can find flimsies (as were used for carbon copies) and your printer will handle them, so much the better.
Because the printed side is in contact with the PCB, light doesn't bleed round the edges of the black areas to any appreciable extent. As I said, a bit of improvement with black ink helps, especially if your printer (like my old b/w laser one) produces a rather streaky black. Another trick is to make two prints, shifting the image a bit before doing the second one so any streaks come in different places, and use one on top of the other after taking care to ensure the patterns register. But having two layers extends the exposure time considerably.
Andy