The correspondence consists mainly of the incoming letters addressed to the firm of Blackwood. It contains also letters passing within the firm between members of the family, between the family and their employees, and between the employees themselves.

The correspondents include authors of the standing of James Hogg, John Galt, John Gibson Lockhart, George Eliot, Lord Lytton, Anthony Trollope, and Joseph Conrad; but the majority were not writers of comparable distinction, nor were all of Blackwood's correspondents professional authors. Several were notable men of affairs for whom authorship was a secondary occupation; many others were men of various professions – clergymen, soldiers, lawyers, and colonial administrators, for example – who wrote, especially with a view to publication in “Blackwood's Magazine”, on matters they felt competent to deal with. As a result, Blackwood's incoming letters cover a great many topics and are a source of information almost as much for the historian of society as for the historian of literature.

As a rule only the writers of letters have been indexed, the recipient being regarded as the firm itself, even when letters are addressed to individual members of the family, to the Editor of “Blackwood's Magazine”, or to employees such as J M Langford, G Simpson, T Henderson, and A Allardyce. Letters not directly concemed with Blackwood, which were presumably enclosed with other letters to the firm, have been indexed under both writer and recipient wherever possible.