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If you clean them immediately, and you can thin them with a bit of water if it seems too thick for the steel brush tool to handle.

A lot of inks just plain spoil if they are sitting for a long time, similar to milk souring. You could attempt to thin it (with water, since acrylic is water-based) and see if it helps.

Upon asking the people at Speedball this question, they told me it will show up on dark paper, but you may have to put on a second coat. We do have a marker that's called the Pilot gold and silver pens that are actually paint in a pen. We have them by Sharpie, as well (Sharpie paint pens). These will work on black paper, really opaque, but you may be wanting to brush it on, or use it in a pen of your own. Another trick some artists use is to mix gold gouache with a little water, to make an ink like consistency, and that is very opaque. A friend who was a calligrapher did this with her calligraphy pens.