Thea,
VERY interesting question! I find myself in a strange position in that I am usually a one-reason/cause man (i.e the Civil War came about because of slavery, zip, period, nothing to add!).
The subject you bring up forces me to consider a VARIETY of reasons as to why the South was defeated, the blockade being only one of them.
Much was against the South, in the way of resources, when the war began. Manpower and manufacturing are two that you have mentioned in your above post. These shortcomings were compounded because of the blockade, but remember, early on the blockade was a joke. Hardly any ships in the Union navy, a vast area to patrol, swift blockade runners getting through almost at will during the first part of the war.
But, due to mistakes in management of the war is when the blockade began to have a more serious result later in the war. Refusing to convert cotton production to food production was one. States refusing to share resources was another. And some of the cargoes carried by the blockade runners were items that could not be used in the war, so-called luxury items that could make huge profits instead of aid the war effort.
So, no, I don't think the blockade was the primary reason the South lost the war, but it in conjunction with many other problems in the South helped lead to its defeat.
Sincerely,
Unionblue