Buy the white decal paper just for this purpose! Its not expensive for 5 sheet A4 packs on ebay it will last for years! This allows you to print whatever you want - names, plimsol marks, depth marks for the bows etc and of course at any size you want - depending on how good your printer is. I have a colour laserjet, the print is fantastic, but you can get good results from ordinary inkjet printers.
I'm this week running off circles with a 'F' inside them to go on the fronts of fire hose cupboards. The reason for using white instead of the transparent is obvious. On the white paper 'fill' the background colour, in your case black, easily done in microsoft Word or other similar progams, then change the text colour to white job done! The transparent paper I began with thinking it was easier to print what I needed, then have the background colour show through the transparent part, ok for some colours but not white. Also I found with strong background colours like black or other dark colours, it makes the decal colour look darker than you want.
So now I use the white paper and an app downloadable to your phone, it works like this, you need the white for lettering, but you need to match the background colour on the parts of the decal around your lettering. The app is a colour grabber, point the phone at your chosen background colour, press the button and it give you the hex code and rgb code numbers for the colour. All you do then is 'fill' the background colour of your decal to match these figures in Word, thne print your text in white again job done.