Since Blake was caught and made to work for us, it would be unwise to let him stay alone.
After all, we know that Excalibur would definitely want to take him out because he snitched on them, so we thought it would be safer for him if he stayed where we could watch him.
And since Edgar and his boys and some other of the gang members refused to take him under their roof, we had no choice but to take him under our roof.
Nico said he was actually quite relieved that he ended up under our care because he felt we would protect him better than the others. He also thought it would be a good opportunity for us to keep an eye on him in case he ever thinks about betraying us.
Also I asked Harry about the symbol and he confessed that we were really a gang but not the typical gangs out there which deals in drugs, theft and other shits like that. We only deal in making and sell