Docluv2run You cannot mix acrylic and oils: chemistries are different.
liquid black: check articles section, somebody wrote about liquid black. I had article about liquid white. However liquid black is a bit trickier. Check what pigments in the Bob Ross liquid black ( it may actually be mix of blue and brown, green and alizarin) add oil to it and thinner. If you just use a lot of thinner it will not have covering power to make it super dark. Proportions of oil and thinner : work out on your own as it will depend how long you paint and how long you need it to stay wet. More thinner - means faster drying time and sinking into the canvas. You just need to try and see what works for you. I would say premix oil and thinner 1:1 and start adding to your paint few drops at a time and see when you get the consistency you like and can manage on canvas.
In oil painting: there is no right or wrong way as there are different techniques - it is understanding of your materials and making your supplies to help you achieve your objectives with less stress. Only solution is to go and try it out.