Yes, possibly you did boil it too long and too much of the liquid got boiled off, making it very stiff. Possiby adding a little water will help. Suggest you just try this with one pot of jam at a time and just add around tablespoon of water to the saucepan and heat until the jam has gently liquified and then return it to the jar which you've washed and sterilised again.
I recently made some Damson jam. This has set VERY solidly, possibly because there isn't much juice in damsons but as the flavour is supurb I've decided to leave well along. Did you add the same amount of liquid to your second batch of blackberry jam?. Possibly if the blackberries were less ripe than the first batch they may have yield less juice which could be a possible cause of the thickness. If you're worried about the jam not resetting a second time after melting it, add a good splash of lemon juice which should help incease the setting ability. Good luck !