The only one who had a weapon drawn was Ja (baseball bat). Say to that what you will, but 50 Cent was the one who yelled to his original DJ in the back (at least I believe it was him) to "grab the gun". 50 Cent confirms this himself. Black Child drew his weapon to prevent G-Unit from accessing a gun, a situation that was made more intense with the lights off. It makes perfect sense and anyone would have done the same in his situation.
Just to correct you -- the weapon Ja Rule allegedly used on 50 during the hit factory incident was a pair of crutches, and he also picked up some speakers and slammed them on fif. This is what allegedly caused 50 to yell, "get the gun". At this moment, Black Child started to stab ppl up. I have never confirmed it, but I think it's highly likely that the lights never went off, and Black only made that claim for legal reasons. I say this because
1) Ja rule and Fif never claimed the lights went off during their retelling of the story. Ja rule even goes out of his way in his book to imply that the "lights off" never occurred.
2) Black child explicitly told Mtv, circa 2003, "I saw 50 cent going for his gun and I poked him".(
src)
3) He only claims the lights went off during the beef DVD, in which he also claims that Fif's "unknown" friend said get the gun, and not Fif himself. This contradicts his and Ja rule's claim that 50 said "Get the gun". Which leads me to believe he is trying to avoid snitching on himself and Fif.
4) 50 cent and Yayo are recorded on the order of protection stating that Black Child had stabbed them, and when Black Child was arrested for the incident, he reports that the police told him, that they(G-unit) said he committed the stabbing. How would they know that if the lights were off.
In conclusion; it seems more likely that Black Child was trying to keep himself safe from scrutiny of the law during the beef DVD, so he retold the story in a way that kept himself and members involved safe from law enforcement.