This is a new engine, right?

Then compression should/will get better when you get some time on the engine. It needs to run in.

Do you mean it is blowing back black fuel into the tank? That is not uncommon either, especially if you are using castor based fuels.
Also the black stuff indicates that the engine is running in. It is metal shavings from inside your engine mixed with oil. The oil is supposed to carry the metal particles out of the engine.

How are you starting it? Are you using a starter, or your hands?

Try this: Choke the engine until it is good and wet (no glow current please). After you get it good and wet, try to turn it over by hand to see if it is not hydro locked because of too much fuel in the combustion chamber. If you are unable to turn it over by hand there's too much fuel in the engine. If this happens, just rock the prop back and forth against compression to clear out the excess fuel.
After you have made sure the engine is wet, but not hydro locked, switch on the glow current and give prop a good hard swing against compression, but in the "wrong" direction. That means clockwise when standing in front of your plane. The prop should bounce against compression, and the engine will start running in the right direction.
This may take some practise, but it works very well. Just make sure your finger are clear of the prop when it hits compression... or they will be sore for a while

For the first minutes, keep the engine blubbering rich and leave the glow current connected to keep it running.

After the first tank or so, GRADUALLY start leaning out the needles, so that it stays running without glow current.