The mere presence of a weapon is probably not, on its own, justification to shoot the possessor. Ultimately, the actions will be judged under the totality of the circumstances related to the shooting. I want to be as neutral as possible here in sharing the information that has thus far come to light but nowhere has "the presence of a melee weapon" been used as justification for the police shooting. This shooting may be found, after an investigation, to be unjustified or it may be found to be justified. Here is some of the information available.
The report issued by Wisconsin Attorney General Joshua Kaul has additional information:
Kenosha Police Department officers were dispatched to a residence in the 2800 block of 40th Street after a female caller reported that her boyfriend was present and was not supposed to be on the premises.
Although the statement from the Attorney General doesn't mention a restraining order filed against Mr. Blake, the criminal complaint filed by an Assistant District Attorney on July 7, 2020 seems to indicate that Mr. Blake had waived the necessity of a restraining order and agreed to stay away from the property. An arrest warrant had been issued based on the criminal complaint. Police officers are trained to be extra cautious when approaching individuals subject to felony arrests.
The Attorney General stated:
During the incident, officers attempted to arrest Jacob S. Blake, age 29. Law enforcement deployed a taser to attempt to stop Mr. Blake, however the taser was not successful in stopping Mr. Blake. Mr. Blake walked around his vehicle, opened the driver’s side door, and leaned forward.
There are reported witness statements from Raysean White that the police repeatedly told Mr. Blake to "drop the knife." No witnesses, that we currently know of, have come forward to say if Mr. Blake was holding the knife that was reportedly found on the floorboard.
Also from the Attorney General report:
During the investigation following the initial incident, Mr. Blake admitted that he had a knife in his possession. DCI agents recovered a knife from the driver’s side floorboard of Mr. Blake’s vehicle. A search of the vehicle located no additional weapons.
Under Wisconsin law, it is a felony to resist arrest while armed with a dangerous weapon or threatening to use a dangerous weapon whether or not the arrestee has a dangerous weapon.
All of the above information, along with any other information gathered from the investigation being conducted by the Wisconsin Division of Criminal Investigation will be used to determine if the use of deadly force was reasonable. It would be unreasonable to draw a conclusion either way regarding the use of deadly force with the limited information available to us. It is not unusual, though, to see the use of deadly force, or the threatened use of deadly force, when an arrestee physically resists arrest.
Your ultimate question as to whether or not seven shots were justified is much more difficult to answer. If we assume that the use of deadly force was justified, a fact currently undetermined, it is much more difficult to say that too many shots were fired. A properly trained police officer will be taught to shoot until the threat justifying the use of force is neutralized. The Attorney General's report states:
While holding onto Mr. Blake’s shirt, Officer Rusten Sheskey fired his service weapon 7 times.
and further states:
No other officer fired their weapon. Kenosha Police Department does not have body cameras, therefore the officers were not wearing body cameras.
It is possible that only Officer Sheskey knows if seven shots were justified. It's possible that Officer Sheskey was affected by adrenaline and shot more times than necessary and it's also possible that Officer Sheskey shot until he no longer perceived Mr. Blake to be a threat.
It is not possible, today, to answer your question about the reasonableness of seven shots and it's possible that it will never be answerable.