There could also be themes of sacrifice. For example, Blackfire might have to sacrifice her own goals or allegiances in order to be with Robin. Or Robin might have to make difficult choices regarding his relationship with Blackfire and his duties as a hero.
One common theme is the clash of different cultures. Robin is from Earth and Blackfire is from another planet, so their cultural differences often play a big role. Another theme is the idea of forbidden love, as they are often on opposite sides at first. For example, Blackfire has been an antagonist in some cases.
You can try looking on fanfiction websites such as Archive of Our Own (AO3) or FanFiction.net. They usually have a wide range of fanfictions including ones about Blackfire loving Robin.
One common theme is the struggle with cultural differences. Since Starfire is from Tamaran and Robin is from Earth, they often face misunderstandings because of their different upbringings. Another theme is the idea of forbidden love, as they are teammates and sometimes there are rules against romantic relationships within the team.
A common theme is the exploration of their cultural differences. Starfire is an alien and Robin is from Earth, so many fictions play with how they bridge that gap in their relationship. For example, how they communicate their feelings when their cultures have different norms regarding love and courtship.
Another plot could be that Blackfire comes to Earth seeking refuge from some intergalactic trouble. Robin is the one assigned to protect her. At first, Blackfire resents the protection, but as time passes, she realizes Robin's good intentions and they form an unexpected bond. Their adventures on Earth, with Robin showing Blackfire the beauty and uniqueness of the planet, could be really engaging.
I'm not sure specifically about 'robin and blackfire lemon fanfiction' as it might involve adult - oriented scenarios which are not suitable for general discussion. But if you just want to read about Robin and Blackfire's interactions, you can try some mainstream fanfiction sites. Some stories might focus on their adventures together, or how they deal with each other in different situations, like facing a common enemy or working towards a shared goal.
One common theme is the clash of different worlds. Naruto is from the ninja world and Blackfire has her own alien background. Another theme could be the power dynamic, as both are powerful in their own ways.
One common theme is redemption. Often, either Red X or Blackfire (or both) are on a journey to redeem themselves in some way. For example, Red X might be trying to break away from his criminal past and Blackfire could be seeking to make amends for her actions on Earth.
One common theme is friendship. Naruto is known for his ability to make friends, and in fanfiction with Blackfire, they might form an unlikely friendship. Another theme could be power - sharing. Blackfire has her own set of powers, and Naruto with his ninja skills, and they could exchange knowledge about their powers.
Well, typically in such fanfiction (assuming it's not adult - related), you might expect a more intense and perhaps angsty relationship exploration between Robin and Blackfire. It could involve their different personalities clashing or meshing in unique ways. Maybe there will be a focus on how they deal with their respective powers and how those play into their relationship.