I just started a batch of blackberry/cherry wine.

I modified a recipe from Jack Keller's website to include cherries.

a.. 4 lb. blackberries
a.. 2 lb. Sweet Dark Cherries
a.. 2-1/2 lb. granulated sugar
a.. 1/2 tsp. pectic enzyme
a.. 1/2 tsp. acid blend
a.. 7 pts. water
a.. wine yeast and nutrient

Ok so here are my questions.

1. I was not able to get a gravity reading, the hydrometer would was
sinking to the bottom. The must had a very sweet taste and that's all I
know. I used a blender to liquefy the fruit. I like for my wine to finish
dry at around 13.5 percent. How can I make sure that the amount of sugar is
correct if I cant get a reading.

2. After deciding to make the wine I thought I would look on the internet
to see if anyone had posted about Blackberry&Cherry wine and I couldn't find
anything. I hope this isn't a bad mixture. Has anyone here done this
before and if so how did it turn out?

Thanks,

David