Are these berries poisonous?

It sounds like you have a Mulberry tree growing in your backyard. Yes they look like blackberries. They are not poisonous. They don't taste bad, but they don't exactly taste good either. The juice from the berries will stain anything they fall onto. This will grow into a medium sized tree in about 15 years.(makes good shade). The birds like the berries,and the birds may have "planted" the one in your yard.