hi goodfella,
I make a lot of small 1 gallon fruit (juice) wine batches, and the occassional kit wine. (I saw your post about 'what to do with all the wine' LOL I have that problem too, so I make many small batches instead of many larger batches)
I started with the "fruit wine kit" from George - gallon jug and a 2 gallon bucket, airlock, tubing to rack with, and the assorted additives, and an acid test kit. I use(d) the acid test kit a lot - got a ph meter eventually (which was a dodgy meter so...) now I am pretty much "go by taste" but will test if I am in doubt.
Like other posters said, you cant just use all fruit and no added sugar for SOME fruits because of the acid content will be way too high. Rhubarb for example is high in acid, and so are some native/wild concord type grapes - especially if harvested too early and not fully ripe (like up here, the frost comes before the full ripening some years ).
Grapes have that natural balance of sugar, acid, nutrients, and flavor plus natural tannins. Other fruits, you have to adjust one or more of those items - usually calls for adding nutrients and tannin and reducing acid and increasing sugar.
Not having made a straight "fruit wine" on the whole fruit and nothing but the fruit (...dried elderberries though I did that one...and one with a melange of stuff tossed in with chokecherry pulp...) I can't advise on the blackberry port per se, but welcome to the world of Fruit Wines! Yum Yum.