In his poem about London, William Blake cries out against privilege, rank, wealth, church and establishment on behalf of the poor, the destitute and the enslaved. I think you are definitely on the right track, and some deeper research into the hsitorical context of the poem set in the filthy city streets of London at that time would benefit you even more. The words 'mind-forged' are reminiscent of two things. Firstly, the word 'forged' suggests eternity and the world of manufacturing, technology and industry - such as that in a blacksmith's forge. Eternity is suggested by the lasting 'weld' or 'imprint' that forging gives. Blake suggests that 'manacles' are forged for ever in these people's minds, as well as in their daily environments and experiences. The hurdles, blocks and challenges to progress, either spiritual or practical, are a permanent mental block of disadvantage.