Naturally, he assumes that the dead man is Mr.
Blackmore--who, by the way, had hinted at suicide only the night before.
With this idea he enters the chambers and sees a man a good deal like Mr.Blackmore and wearing Mr.Blackmore's clothes, lying on Mr.
Blackmore's bed.

The idea that the body could be that of some other person has never entered his mind.

If he notes any difference of appearance he will put that down to the effects of death; for every one knows that a man dead looks somewhat different from the same man alive.
I take it as evidence of great acuteness on the part of John Blackmore that he should have calculated so cleverly, not only the mental process of the porter, but the erroneous reasoning which every one would base on the porter's conclusions.

For, since the body was actually Jeffrey's, and was identified by the porter as that of his tenant, it has been assumed by every one that no question was possible as to the identity of Jeffrey Blackmore and the tenant of New Inn." There was a brief silence, and then Marchmont asked: "May we take it that we have now heard all the evidence ?" "Yes," replied Thorndyke.

"That is my case." "Have you given information to the police ?" Stephen asked eagerly.
"Yes.