In both stories, suspense is created by repetitive images and sounds. In "The Tell Tale Heart," the narrator hears the beating of the heart constantly, like the ticking of a clock counting down the seconds. Poe writes in an "arabesque" style, in which he is incredibly detailed and dramatic, for example: ‘‘I heard a slight groan, and I knew it was the groan of mortal terror. It was not a groan of pain or of grief—oh no!—it was the low stifled sound that arises from the bottom of the soul when overcharged with awe.’’
In the black cat, the image of the hanged cat that is left on the burnt building and the markings that look like a noose on the new cat are repetitive images of death and the murderous deed that the narrator committed. Like the endless beating of the victim's heart in "Tell Tale Heart," the constant reminder of the death of the cat through the images of nooses and cats with one eye remind the narrator of his evil deed. The narrator blames the cat (in his mind melding together the two cats into one evil cat) "whose craft has seduced me into murder."
I'm not sure about the Black Cat, but we've been doing the tell-tale heart at school. The fact that we don't know any details about the setting, the narrator or the victim builds tension because it makes it seem closer to home.
The narrator seems to be mad, because he kills this guy for no reason. This builds tension because the reader doesn't know what's going to happen next; they can't comprehend his (or her) mindset.
The repitition, capital letters and exclamation marks add shocks and make the reader jumpy.
The whole thing about the eye, and the fact that everything happens at midnight etc. builds suspense because modern kids are taught to fear 'the witching hour', ie. midnight, and 'the evil eye', from the time when they first start to hear fairy stories. Poe uses this to make the reader feel apprehensive.