If Darcy had only given his opinion to Bingley, I wouldn't blame him, but he did more than that, he was "earnestly" determined to "convince" Bingley -- re his letter --
"I readily engaged in the office of pointing out to my friend the certain evils of such a choice. I described, and enforced them earnestly. But, however this remonstrance might have staggered or delayed his determination, I do not suppose that it would ultimately have prevented the marriage, had it not been seconded by the assurance, which I hesitated not in giving, of your sister's indifference. He had before believed her to return his affection with sincere, if not with equal regard. But Bingley has great natural modesty, with a stronger dependence on my judgment than on his own. To convince him, therefore, that he had deceived himself, was no very difficult point."
As for the concealment of Jane's presence in London, it was because Darcy thought that Bingley might be in "danger" if he saw Jane again i.e. that Bingley might wish to pursue Jane again if he saw her --
"That they might have met without ill consequence is perhaps probable; but his regard did not appear to me enough extinguished for him to see her without some danger."