I've never been much for blackberries because I go for boysenberries based on availability (and flavour that's more than just sugar). With that said, though, berry ciders are great and I could see hops complimenting them well if used correctly. Off hand, I think that your biggest challenge will be managing bitterness and acidity. Coincidentally, I'm also playing with the idea of a hopped fire cider myself, possibly with berries, but as full disclosure I've never actually done a hopped cider so I can't give you real notes on the hopped bit.
I think you should develop this idea and look at doing it. I'm 100% here for it.
You probably have your own system worked out for the berry / fruit bit, but if not here are my thoughts:
For additions you're looking at three main options:
- Whole berries
- Berry juice
- Organic or non-organic flavouring agent
I recommend looking at the latter two options if you don't already have your own system for this worked out, and here's why:
I'm thinking about this in the same context as I would for a mead - Whole fruit additions in secondary are totally doable, but I'm not a fan of it because it requires continuous monitoring for flavour and takes a relatively long time compared to the other two methods. This is totally a personal quirk of mine but when I add flavouring agents I like to make the addition / play with it if necessary and then be done with it so I can forget about the project in secondary without any loose ends. Not a bad option, though.
I would recommend using berry juice if this is at all practical for you. I need to post my notes for doing so, but you can turn a pressure cooker into a steam-juicer and get plenty of berry juice out of 3-5 pounds of frozen fruit. This is usually what I use to add fruit flavours to my projects, and I strongly recommend making a cider with some added boysenberry juice if you ever have the chance to.
Lastly a quick note on flavouring agents: When I made my first batches of cider I used boysenberry pancake syrup as an all-in-one back-sweetening / priming / flavouring agent. Obviously the usual rules apply in terms of finding products with no preservatives, made from actual fruit, etc. but I would encourage anyone reading this to not overlook this option. My go-to brand of syrup is Pioneer Valley and I've made some pretty good stuff from it.
TL;DR I think a hopped berry cider absolutely worth pursuing, and berry ciders are the bomb.