First of all, we should note that Bismarck was an important political figure from 1862 until 1890. This means that he could not have been speaking about the 18th century when he predicted that blood and iron would be more important that speeches and votes. I will assume that you are asking about the time from Bismarck’s own time through World War I and perhaps WWII. During this time, it is fairly clear that Bismarck’s prediction was proven to be true for the most part.
What Bismarck meant by this statement was that only war would settle the most important disputes between countries. Things like diplomacy and the creation of treaties could not, in his mind, settle disputes. The time from 1870 through WWII seems to bear this prediction out.
Beginning in the 1860s, Bismarck led Prussia in a series of wars that ended up creating a unified and relatively powerful Germany. In that way, he used “blood and iron” to settle the questions of whether Germany would be unified and powerful. After 1870, there was relative peace. However, a major question was brewing. This was the question of whether Germany would become an imperial power on the level of France and the United Kingdom. This question was not settled by diplomacy. Instead, it was eventually settled by World War I. We can say that the question went on to cause WWII in Europe was well. On the other side of the world, the question of Japan’s right to have an empire in Asia also ended up being decided by WWII.
Thus, we can see that major questions during and after Bismarck’s time were settled by war, not by diplomacy and treaties.