Had it been known in 1917 that Germany had committed an attack of this magnitude on American soil, then the Black Tom Explosion would be more famous now. As it happened, however, the explosion did not immediately alarm the American public to the possibility of German espionage. A large depository of munitions had exploded, killing a few people, but that is one of the natural risks of making and storing munitions: they sometimes explode. Of course, investigators quickly realized that it was not an accident, but it would not be until after the war that investigators became reasonably confident that German espionage had been involved, even though that was their initial suspicion (after all, the Germans had been sinking ships with American passengers in order to prevent American munitions from reaching Britain, e.g., the Lusitania). Unlike in the case of the Lusitania, as far as the American people knew, it had just been a terrible accident

Of course, the Black Tom Explosion is not the only example of German and Austrian espionage during the war; in the same time period, American intelligence discovered that the Ambassador from Austria Konstantin Dumba had made plans to disrupt the production of American munitions by inciting strikes, and in response and the United States expelled him. Of course, Dumba's plan didn't involve blowing up the plants, and in any case he never saw the plan through, so this certainly was not as serious as the Black Tom Explosion, which still stands as the most interesting example of foreign espionage in the United States during WW1 (there's a lot of material on this subject, but it's all pop-history as far as I know; in fact, even in the process of earning my master's degree in American history with an emphasis in diplomatic history I didn't learn about this stuff until I did personal research for a thesis on US-Czechoslovak relations, which exposed me to the history of the Czech-American spy Emanuel Voska).

But I think your other point is just as interesting: how did the Lusitania become intimately linked in historiography to the American decision to enter the war given that sank two years before Wilson asked Congress to declare war? The answer is in two parts: first, the sinking of the Lusitania, a huge passenger ship with over a hundred American aboard, by a German submarine (without warning) was the most famous example of German unrestricted submarine warfare. It made headlines in the United States, remained a scandal for the next few years, and garnered international attention that allowed Britain--and the ship was British--to make a big deal about it as well. Second, the Germans had just announced that they were to resume unrestricted submarine warfare two months before Wilson gave his speech to Congress asking for American intervention and in fact played a role in Wilson's decision to bring America into the war ("unrestricted submarine warfare," if you are unfamiliar with the term, means the sinking of ships by submarines without first announcing one's intention and allowing people to escape from the ship. Since most useful targets came equipped with anti-submarine guns on the deck, it made legal submarine warfare impossible to implement, as it required the submarine to announce its presence and surface). Wilson opened the speech by talking at length about unrestricted submarine warfare, calling it, among other things, "the wanton and wholesale destruction of the lives of noncombatants, men, women, and children, engaged in pursuits which have always, even in the darkest periods of modern history, been deemed innocent and legitimate." Wilson did not mention the Lusitania by name--the German renewed effort at unrestricted submarine warfare, after all, had nothing to do with the Lusitania, which had sank in an earlier, undeclared period of unrestricted warfare--but it was and would remain the one example that most Americans could call to mind. It is unfortunate, however, that we often teach children that the Lusitania caused the United States to enter the war, though I guess this is far better than telling them that Christopher Columbus was the first person to think that the world was round.

I should also note that Wilson does bring up espionage in the speech, although it plays a secondary role to such principles as combating unrestricted submarine warfare and making "the world . . . safe for democracy." I suspect, however, that Wilson would have made a bigger deal of this point had Americans known what we know about the Black Tom Explosion today.