Why do we have a situation where the past of "to blow" is "blew", but of "to glow" is "glowed"? And don't say "flew" if you mean "it flowed". The poem Lovers, by Phoebe Cary has many examples of these.
How did these differences originate? Did "blow" and "glow" come from etymologically distinct backgrounds that have just come to be spelled and pronounced the same way? Is there a general rule for words like these?
The name for the difference is "strong" versus "weak" verbs. Strong verbs have a vowel change in the past, weak verbs add -ed. (Life gets slightly more complicated: for instance, there are irregular weak verbs, but that's not relevant to this question.)
It basically has to do with how frequently the words are used (or were used in the past). We learn to use these other past forms by hearing them repeated. If we don't hear one enough, the strong past form (the one not formed with -ed) sounds "wrong" to us, and we use the -ed form.
You can see this with young children of the right age, who routinely try to use -ed on strong verbs. When a word is used rarely enough, people grow up without ever getting comfortable using the strong past form, and continue to use -ed. They then pass that on to their children, and so on.
So over time, strong verbs become weak if they're used infrequently. You can see this dynamic in play today with words that are on the edge of the necessary frequency; for instance, what is the past of "dive": "dived" or "dove"? Quite possibly in a generation or two, "dove" will sound archaic and "dived" will be standard.
pled guilty sounds a lot better to me than pleaded guilty
The reasons are indeed historical. In Old English, glow (glowan) was a weak verb, while blow (blawan) was a (class VII) strong verb. Wikipedia has a nice section on Old English verbs. One of the key passages is probably this:
The linguistic trends of borrowing foreign verbs and verbalizing nouns have greatly increased the number of weak verbs over the last 1,200 years. Some verbs that were originally strong (for example help, holp, holpen) have become weak by analogy; most foreign verbs are adopted as weak verbs; and when verbs are made from nouns (for example "to scroll" or "to water") the resulting verb is weak. Additionally, conjugation of weak verbs is easier to teach, since there are fewer classes of variation. In combination, these factors have drastically increased the number of weak verbs, so that in modern English weak verbs are the most numerous and productive form (although occasionally a weak verb may turn into a strong verb through the process of analogy, such as sneak (originally only a noun), where snuck is an analogical formation rather than a survival from Old English).
Flow is actually interesting in that it is yet another example of a strong verb becoming a weak verb. In Old English, flow (flowan) was a class VII strong verb, just as blow. (In contemporary German, both are still strong verbs: blasen — blies, fliessen — floss.)
As you can see for yourself, all this leads to a rather sad answer to your last question: no, there isn't a general rule for words like these. You pretty much have to learn them by heart.
Now, how the verb classification in Old English came about is a different story entirely, and I really hope that that is out of the scope of this question. We would have to dig deeper into Proto-Germanic or perhaps even Proto-Indo-European to find an answer, if it can be found at all.
