a drain system is fine but, it NO way will keep all water/moisture away from...out of...a crack.
and, many cracks will widen, get bigger and then allow MORE water/moisture into a hollow-block wall. this is Often why mold/mildew/efflorescence/discolorment will be seen on the inside.....`cos water is allowed to enter.
so, a builder/whoever that isn`t concerned about a crack either doesn`t fully understand 'how `n why water enters' and/or doesn`t care.
I`ll say it again, raising-sloping the grade or longer downspout extensions or french drains etc etc does NOT seal/waterproof any crack, these techniques will divert 'some surface' water away which is fine, but cannot keep all water/moisture away from the entire DEPTH of a foundation wall, and where any cracks are.
And some cracks will appear hairline in size on the inside but, can be wider on the outside...this is just how block walls crack.
Often with block walls, there will be NO 'visible' crack when looking/inspecting the walls on Inside of basement but on the Outside there are hairline to 1,2++ inch wide cracks. And again, THIS is where water/moisture enters INTO a hollow-block wall and will often show up inside at/near where the floor and wall meet...cold joint.
Did the builder damproof the outside of the walls or Waterproof them?
Did they 'Backfill' against the Outside walls with at least 3/4 gravel/peastone?
Or did they maybe backfill with 1' or so w/gravel and then push/backfill rest w/ soil from site? This is what you DON`T want.
That crack 'could' have been caused from the soil compacting/settling last month(s)...it does indeed create pressure against basement walls.
--Carefully read ALL of-- http://www.bobvila.com/ArticleLibrary/Task/Inspecting/FoundationFailure.html
see 6th, 7th paragraphs http://www.yodergroup.com/concrete.asp
Unbiased links here folks....think about what they are trying to inform YOU about. Soil pressures on the OUTSIDE of basement walls...YOUR Foundation walls!
Sure there are other important factors when building houses/walls incl`g site soil testing,footing depth & thickness, using--not using steel reinforcing rod in basement walls,also....what is the Best backfill to have on the Outside of Your basement Walls, which applies the LEAST lateral load & which provides excellent `n immediate drainage along the walls.
There is a SIGNIFICANT difference between damproofing and Waterproofing, oh YES there is!
see first few paragraphs and also 5th P " To permanently Stop water & water vapor from entering your foundation walls, you need to WATERPROOF them. Damproofing is an inexpensive way to meet the 'lowest' minimum standard of the building code"