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.
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.
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.
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.
Starfire and Blackfire in lemon fanfictions can be quite interesting. The 'lemon' part might introduce scenarios where their powers or their Tamaranian heritage plays a role in their relationships. For example, their different approaches to love and power could be the center of a story. Maybe Blackfire, being more aggressive, could have a different kind of relationship dynamic with a character compared to the more naive and kind Starfire.
The appeal lies in the contrast between the two characters. Starfire is often seen as the good - hearted, innocent one, while Blackfire is more of the rebel. In lemon fanfiction, this contrast can be explored in relationships. It gives a chance to see how they would react in different situations, like a love triangle or a power struggle within a relationship.
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.
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.