If you go to admin/structure/block and mouseover the "configure" link you'll see something like this for blocks added through UI:
/admin/structure/block/manage/block/1/configure
something like this for blocks created with views:
/admin/structure/block/manage/views/81e6b321879efa58ba9e546521c14467/configure
something like this for blocks created with webform:
/admin/structure/block/manage/webform/client-block-50/configure
something like this for blocks created by the system (core):
/admin/structure/block/manage/system/main/configure
I've bolded the deltas in each example.