Steps- printing white over black with hand-rubbing is probably the hardest thing you can do with block printing. Most white inks do not carry enough pigment to do it well, and hand-rubbing doesn’t give you enough pressure…. so the end result looks mottled. Speedball ink is especially problematic in that regard. Most printmakers I know would do a print like yours just the opposite: black ink on white paper. As you have discovered, it works better that way.

Now…. since you have already cut the block, what can you do? There are several possibilities (in the order I’d try them):
1. using oil-based ink, you can dampen the paper. That improves the impression, and will help with overall look.
2. with waterbased ink like Speedball, add a little "extender’ to the ink. That will reduce it’s mottled effect a little.
3. Use a smoother paper, and a lot more pressure. If you really want to do black on white, think about getting a press.
3b- use a smaller barren. I use a 1inch diameter cabinet knob to rub with. The smaller surface are will given you more pressure per square inch and improve your impression.
3c- improve your rubbing technique- use a single sheet of paper between your print paper and the barren (rubbing tool) to protect the print, and then rub methodically, and with some pressure.
4. Work on your inking technique. Use a harder brayer, and put a little pressure into it. One of the things I’m seeing in your pic is that the roller is not laying the ink down evenly. Usually this is caused by being too delicate with the roller. However, if you have a soft roller, this can lead to ink where you don’t want it. The solution to this is twofold: a- get a harder roller. and/or b- cut your image and background a little deeper.

Now if you all of these things, will it make your white on black perfect? probably not…. but it will look better.

a timely post
I am overseeing a relief printing project in a school classroom where grade 6 students are printing white ink onto black paper from a carved soft matrix surface using hand rubbing with back of a metal spoon technique.
We have encountered similar problems as you have been experiencing.
A very good paper is being used, in this case it is a thin thai made black mulberry. But the ink is printing blotchy or uneven.
My observations lead to me to believe the following are factors:

The ink being used, a water based Graphic Chemical made product might have pigmentation problems. It could also be that after several rolls with the brayer to pick up and coat it with ink it is starting to dry a bit on the roller. Heat in the classroom is very warm as I live in Northern Canada and it is winter. It is possible the soft synthetic rubber brayers we are using may have pockets of invisible film of oil based residue on many of them (they have been used in many schools for the last 6 years) and are starting to show signs of wear and tear from a lot of rough handling.

I am not familiar with the Daler-Rowney paper you are using. Is this a general purpose paper for various media or is it specifically for printmaking applications?
Learned a while back in my studio that some types of papers do not print well such as highly absorbent cover stocks and construction papers.

I have used white oil-based relief printing inks (Daniel Smith, Faust) hand printed onto black mulberry and have achieved satisfactory results. Believe the key is application of appropriate pressure and a good even coating of ink on the block surface.

Hopefully this thread will share more knowledge learned through experience and observations from others who have either had problems and/or resolved them will help a lot of us.