The Alliance wanted to reduce the reload time of the original Widow rifle, without sacrificing its stopping power. The solution was to increase the number of shots the gun could fire before it needed a fresh thermal clip. Heavy for a sniper rifle, the Black Widow's firepower more than compensates for its encumbrance.
The Black Widow seeks to improve on the M-98 Widow's faults. The result is a powerful rifle, still capable of dealing significant damage, while at the same time giving the Black Widow three shots instead of one before reloads are needed and double the ammunition reserve. However, it does less damage per round than the standard Widow; each shot does the same damage as the M-92 Mantis, which is still very respectable, so the Black Widow can be seen as a blend of the original Widow and the Mantis with a better ammo reserve and three shots per reload.
The main strength is that the Black Widow has much higher damage than any other semi-automatic rifle--it can kill weaker enemies outright in one shot to the body while stronger targets can be picked off with a single headshot. The high damage, combined with the semi-automatic firing means that this rifle can achieve much higher damage per second than most sniper rifles. Against armor it is virtually unparalleled.
The biggest problem, however, is that it weighs as much as the M-98 Widow, making it certain to increase power cooldown times. Because of this, it would be highly recommended to avoid using this weapon on classes that rely heavily on their powers, such as Adepts, Vanguards, or Engineers.
Another problem with this gun is its recoil, thus even with the highest concentration mod you won't be able to squeeze 2 shots onto targets in one section of a scoped mode, as the recoil will eat away time needed to realign for the next shot in time - to compensate switch in-and-out of the scoped mode between shots. It might sound as if it defeats the purpose of having a semi-automatic rifle, but in a trained hand, and with the hitting power of the Black Widow, that still might mean downing 3 targets in quick successions before any need for a reload if you can get headshots all 3 times.
It is recommended to use the Black Widow with Adrenaline Rush. Adrenaline Rush compensates for the loss of damage per shot. You can get off at least 2 clips with the Black Widow while in Adrenaline Rush.
This weapon is extremely powerful in the hands of an Infiltrator with a maxed out Marksman. Combined with Tactical Cloak, a player can kill even the hardiest of enemies of lower difficulties. Even on higher difficulties, most enemies will not survive the 3 shots in quick succession.
The Black Widow does have a steep initial purchase price. However after initial purchase, the cost to upgrade will be the same as any other weapon's upgrade cost (eg: from level III to IV).
With a great deal of spending discipline and completion of all side missions, the player can buy and begin upgrading this rifle before Priority: Tuchanka. It helps to be diligent about scanning all clusters for war assets as soon as they become available on the galaxy map, which will reveal a lot of credits early in the game; taking the store discount option from the remains of a Reaper destroyer on Borr may also be useful. Additionally, remember that buying the weapon from the Citadel is marginally cheaper than buying from the Normandy.