The dilute milk is, in fact, one of the organic remedies for plant mildews, along with baking soda solutions, and dilute human urine (4-1 ratio). Plant mildews live and spread on the surfaces of the plants (leaves stems, flowers etc), and while none of these remedies actually kills of the infection completely, all will act as controls which reduce the severity of the attack and delay the spread. However, they do not in fact produce any form of protective barrier.
Potato and tomato blights are entirely different type of fungi, in that the spores, having landed on the plant, immediately penetrate the surface and take up residence inside the plant where they cannot be attacked by any form of control other than systemic ones (of which there are none for blight).
The present protective treatments for blight are purely prophylactic in that they must be present before the blight spores arrive, they do not form a barrier to penetration, but work by killing or neutralising the blight spores which land on them, and they are usually effective only until the next shower of rain washes them off or dilutes them.
I doubt if dilute milk would have any effect whatever in protecting against potato or tomato blight, although in a really wet summer I doubt whether any of the methods available to amateurs have much effect anyway, and adoption of proper post infection procedures is of more value.
As to realfood`s suggestion, his treatment was many years ago the staple treatment for the Aspidistra, the rubber plant, the `swiss cheese` plant etc, but that was in the days when full cream milk came with an inch of rich yellow cream on the top which could be scooped off and whipped, or poured over the tinned peaches on Sundays, but I haven`t seen milk like that for very many years, and half of you probably won`t even know what I`m talking about.