Rubble would be a terrible idea, even it wasn't limey. You need a water retentive soil but with plenty of incorporated organic matter. As you note, builders rubble tends to be very limey because of the cement/concrete/mortar component. In fact, if your pond has a cement lining, you need to take it away, because the acidic soil will leach the cement into the soil, which the blueberries won't like.

Ericaceous compost can be purchased at about £5 for a 50 litre bag from any garden centre or DIY shed that sells garden stuff, you might pay more at a posh one. I think you can grow blueberries in it neat as it comes. But unless you have limey soil where you live, you could mix a bit of garden soil in with it to save money.

If you chipped your sycamore you could use it as a mulch on top of the soil you grow your blueberries in. If you want to use it as the growing medium, you would need to compost it first, which means chipping it and then leaving it to compost for a year or two. Wood chips do indeed tend to form an acidic (ie ericaceous) compost. Not as acidic as pine needles though.