First of all the melon will stop growing. Then start checking the tendril [curlicue] on the vine near the stem. When ripe it will turn brown. Wait about 5 more days for most varieties. The melon surface will get more dull also when ripe. On larger melons the spot where it lays on the ground will turn creamy color when ripe.. on small nelons that may be less of a sure indicator. Large melons will have a certain sound when just the right ripeness...less certain on iceboxers.

Hope this helps.

Thanks wayne, now I know what to look for.

I'm also growing blacktail mountain for the first time, and am having the same problem. I picked one melon that had the brown tendril and a yellow bottom and it was ripe,
then I picked another with the same signs, and it was almost all white inside. Go figure.

I grew black mountain last year and am growing it again this year. A good tasting watermelon, though not as productive as some varieties. The melons ripen much earlier than most and do not get really large; I'd say about 8lbs. I did as others above have said, looked for the brown tendril and yellow on the bottom. Also the sound test. Start tapping them now to get a sense of how the pitch changes -- you should hear it starting to get lower until it is more of a thud. (I once read somewhere that a ripe melon will be about a D pitch, so get out your tuner! LOL)

Blacktails and Sugarbabies have a bright yellow bottom from the start so don't go by that clue. The yellow bottom clue works well for larger white bottomed melons. Besides the brown tendril and other good clues posted above another one I find useful (esp for small melons easy to turn over) is to look for tiny black specks on the bottom that don't rub off. These are mildew specks and indicate that the melon has sat on the ground for a long time, long enough to be ripe. These black dots show up after the tendril has browned. The black specks are very small (smaller than this period . ) and are found grouped up in a patch of dots. If you ever buy a melon at the store it's the one clue to look for (within the yellow bottom) since no tendrils are attached.