Soil conditioner is just another name for garden compost, manure or spent mushroom compost, all of which will improve the condition/structure/fertility of your soil.
Garden centres sell bags of compost labelled 'soil conditioner' to differentiate between it and the various types of potting compost.
Black Gold is just a brand of compost with soil added.
Farmyard manure is what it says it is - if you're buying it in bags from the garden centre it should be well rotted when you get it and it can be dug straight into your soil - it's my first choice to improve my soil!!!
You can also buy 'spent mushroom compost' (look online) and this will also improve the condition of your soil - however it will contain lime so if you garden on acid soil and wish to grow acid loving plants it's not a good idea to use it.