Welcome to BB Catriona.
Regardless of it being fly strike, I recommend you take your rabbit to the vet for formal assessment. Even if it’s not fly strike, only a rabbit professional will be able to treat causes of a “black lump”. Fly strike signs are usually near the butt (where poo exits).
You did not ask about this, and I’m glad to see you are getting your rabbits spayed, but to make sure they bond properly, they need to have a good foundational relationship. If one is biting the other, this will heavily impair the ability to bond them (imagine trying to befriend someone who constantly hits you on the head). Rabbits who are not fixed need to be separated to prevent fights, pregnancy, and grudges. This is not only for happiness, but health as well. The black lump could very well be serious damage from the other rabbit’s bites, in which case your two rabbits will have an extremely difficult time bonding with this bad air.
They need to be separated permanently so they forget each other. I recommend reading the bonding information on this site to learn the basics of bonding, which all apply after the spays occur and you wait a month after to let the hormones regulate following the spay. https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/9/PID/940/Default.aspx
Young rabbits will get along pretty well because they are not very hormonal, but once the hormones to develop, they translate into fighting and biting, which can escalate very suddenly. This is hard to imagine because owners may see their rabbits cuddling and grooming each other sometimes, but a true bond is 24/7 no fights and both rabbits being fixed generally. An unstable bond, where biting occurs, can lead to bullying and depression in one or both rabbits involved.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.
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