This is a good site with paddling animations to learn the basics, such as how not to use the paddle upside down. http://www.kayakpaddling.net/
Everyone I know who learned how to kayak on their own seems to hold the paddle upside down -- I even had friends who "had been paddling for years" hold their paddle the wrong way.
I am also a big, "muscular", guy with big feet and own a Dagger Blackwater and Axis. It is a good choice for those of us looking for something between all the kayaks made for sticks and the almost a canoe cockpits.
As far as foot placement, I place the ball of my foot on the foot pad with my ankles resting on the bottom of the kayak -- so my feet kind of make a V. Don't wear sandals, they get stuck on pads, hurt your feet and usually get lost in the mud. I also like the NRS paddle shoe for the extra ankle pad on the outside -- http://www.nrsweb.com/shop/product.asp?pfid=2307&pdeptid=1169 . For the way I rest my feet that extra padding really helps. I have tried foam pads in the kayak but they never last long.
Regarding sitting in the Blackwater. You want to sit with your knees bent up so that your legs make contact with the thigh/knee pads under the deck. At your size you should really fill up the cockpit and easily be able to contact the kayak with your thighs or knees. The Dagger Axis has similar cockpit dimensions and they make a thigh pad kit for it, which should fit the Blackwater. If you are thinking of getting this let me know and I can check out my Axis thigh pad against my Blackwater.
Also for the Blackwater seat consider dropping the backrest down as low as it goes. Also you may want to detach the bungee that pulls the backrest forward when you aren't in the boat, as this makes it a lot harder to get back in the cockpit. The ratchet band on the right of the cockpit does a good job of keeping the back rest from going back to far and popping under the combing.
I would also recommend placing something under your thighs to support them in the knee up position. A towel, pool noodle, or a kayak thigh rest all do the job.
The Blackwater with the skeg up can really spin. It makes it very maneuverable when needed. Once you learn more how to paddle you will easily be able to track the kayak straight. On lakes the skeg is more useful but on twisting narrow streams you will love the kayak with the skeg up.
I have a pre-packed emergency dry bag for every trip. Small med kit, bug spray, Swiss army knife, compass with mirror, couple of power bars, matches, manual flash light, and a towel (always bring a towel). Usually in the cockpit have throw bag (which is used almost every quiet water lake trip to tow someone back to shore who screwed up), a paddle float (never used outside or practice), water bottles, and a dry bag with snacks, waterproof camera, waterproof binoculars, and cell phone in waterproof box/bag. The a manual bilge pump is good to have but most of your trips now should be on mellow protected water near shores (the Blackwater should never go out to sea) it is usually easier just to go to a bank, flip it, and then use big car sponge to get the extra out. You might just want to get one of these for mellow trips as it can be used as a pump if needed and water gun when not.