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 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.
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.
To write this kind of fanfiction, start with a strong plot. Maybe a threat to the Teen Titans that forces Robin, Starfire, and Blackfire to interact closely. As for the 'lemon' aspect, it could be about exploring their unspoken desires. Starfire might have a hidden longing for Robin that she doesn't fully understand, and Blackfire could use this to her advantage. Write detailed descriptions of their emotions and the charged atmosphere between them.
One idea could be a story where Robin is caught in the middle of a rivalry between Starfire and Blackfire, and there are some 'lemon' - like tense and passionate moments. For example, Blackfire might try to win Robin over just to spite Starfire, leading to some very emotional and charged scenes.
It can start with a common goal. For example, they both want to stop a certain group of criminals. Working towards this goal makes them interact more, leading to the development of their relationship.
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.
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.
Well, 'A Chance Encounter' is a really interesting 'Robin x Blackfire fanfiction'. It starts with a chance meeting between them that sets off a series of events that gradually draw them closer. It has some really great character development for both Robin and Blackfire. There's also 'Unlikely Allies' which takes a different approach and shows how they have to work together despite their differences, and this cooperation slowly turns into something more.