At the quantity you specified for your recipe, I'd say that you're pretty light if you're looking for any significant blackberry flavor in your melomel. Having said that, I don't know how well your ale yeast will ferment or what sort of flavor profile it will produce so I'm hesitant to offer specific advice on your fruit additions.
The thing I like about wine yeast is that they're designed to produce a known result in wines and mead that more closely resemble wine than ale. I know what kind of profile almost any given variety of wine yeast will provide and to what degree they'll ferment with a specific amount of honey, sugar, or fruit. That's why I'm hoping someone who has made mead with ale yeast will jump in and offer a suggestion.
I might want to add that, when using a straining bag, especially in the secondary, you should add some sanitized marbles, or something similar, to keep the straining bag submerged and the fruit wet in order to avoid spoilage and off flavors. You don't want to stir in the secondary in order to keep oxygen introduction to a minimum. Stirring and punching down the fruit in the primary's fine but not desireable in the secondary, so submerging is the answer.