gayle, I understand that you won't be able to get any black as not meaning it isn't available, but you aren't fooling with it for this project...been there, done that. Use the same color embroidery in the bobbin as you are using on the top. I would reccommend you change bobbin color to match top in case everything doesn't sit exactly right and you won't have black dots on top.;)
I did embroidery on a black apron myself and used light bobbin thread for the light colours and black bobbinthread for the darker colours. I would sugest serger thread if you are not able to get black bobbinthread. Good luck
Gayle, I am not sure why you can not find the black bobbin thread - but in a pinch - you can always use regular thread or the same thread in the top and the bobbin. But be careful - with the black on the back of the apron - that would look great from the back - but the bobbin thread could show through on your colors, too - so test this out and make adjustments - so you are not unhappy with the finished project. *4U
We can get black bobbin fill in Australia, Coats bobbin fill.
But why are you using black in your bobbin? If you embroider the apron in light colours you will see the black bobbin fill in the needle holes.
it's strange you cannot find black bobbin thread, we have it here in holland.i think common sewing thread will be to thick.so in that case i should prefer embr.thread.
I have used black serger thread when in a pinch. Before bobbin thread was popular for embroidery serger thread was often used. It is a bit lighter in weight than all purpose sewing thread. Hope this helps.
If you are not using metallic threads, I would suggest using the same thread in the bobbin as on top.