How could a person potentially overdose on negatively inotropic drugs such as beta blockers etc. And how does this effect the heart?

Too much/too many. If you take too much of anything (including water), you can overdose. Od of a bb will cause low blood pressure, excessively slow heart rate, heart block, dizziness and passing out. Enough of anything can cause death. Don't consider it for suicide: it would be unpredictable and unpleasant.