since the gold dragon has a human form, will the black dragon have its human form too?

He technically does have one already but he said he would never be one I believe

The gold dragon's form should be an elf but the artist took some liberties. Raon our black dragon has never taken a human form with in in the novel.

Slight Spoiler

It’s been a while since I’ve read the novel so take this with a grain of salt, but Dragons have different stages that they need to pass to become full adult dragons. In the first phase they get their attribute (like how Eruhaben is the gold dragon) and I believe it’s their second phase where they get the ability to have a humanoid body. I could be wrong and it might be the third. But as of this recent manwha chapter Raon has not gone through any of the growth stages.

If I remember correctly, it was brought up not that long after they rescued him and started traveling with him. He said that he would never take on human form. I think he said that largely due to his trauma. That's largely why he just makes himself invisible instead of taking on human form.