One peculiarity of the Sahidic dialect of the Coptic language is its use of supralinear strokes, which is a fancy (and vaguely obscene-sounding) way of saying simply "lines above letters." These lines can appear over a large number of consonants, but the most important ones can be remembered with the blemner mnemonic.

Otherwise meaningless, the word "blemner" strings together the consonants B, L, M, N, and R. These are all voiced continuants, which is to say, consonants whose sound can be held indefinitely. The reason B is on the list, by the way, is that the ancient Coptic pronunciation of the letter was probably closer to the sound of our letter V, just as modern Greek makes a V-sound with its letter beta.

Continuant consonants can, in a way, function as vowels. If you think of the last syllable of a word like sudden or butter, especially in U.S. and Canadian pronunciations, you'll notice that you don't really need to use a vowel to attach that final sound to the rest of the word. Strictly speaking, there isn't even a schwa there; all you've got is the approximant consonant doing its work.

So far as linguists have been able to determine, the line drawn above blemner consonants in Coptic signalled that the letter would have been pronounced this way. Good thing, too, since Coptic words sometimes contain an alarming number of consonants smashed together, without enough (or any!) vowels to support them.
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BBRRE: we hear
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KRMRM: to mutter

The supralineal stroke which graces the blemner consonants in these words instruct the reader to make a syllable out of the letter in question, just as you might if you saw the English word "muttr." (As an aside, KRMRM for "mutter" is one of my favourite onomatopoeic words in any language, right up there with the French ronron, for "purr.")

Although Coptic is an Afro-Asiatic language, and thus distantly related to Semitic languages like Hebrew, Arabic, and Aramaic, it should be noted that its alphabet is not an abjad: it does include vowels. Some Coptic words, in fact, seem to possess a surfeit of vowels, making them just as alien-looking to an English speaker as the consonant-choked examples I listed above:

NEUHIOOUE: their roads

EIE: then

Creative uses of semivowels and glottal stops should help you pronounce these words, or so my textbook tells me.

Not all consonants with supralinear strokes are blemner consonants, and linguists are not exactly sure how the exceptions were pronounced. Modern scholars of Coptic will insert a schwa after the stroked consonants in a word like:
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CPCUPF: to entreat him

In some Coptic texts, a stroke will appear where the reader might expect an E (epsilon, the letter borrowed from Greek). In others, an epsilon might appear where there is usually a stroke. Like all ancient languages, Coptic spelling was not standardized even within dialects, and the reader should expect surprises. Nevertheless, blemner consonants seem to have specific rules governing their stroking (ahem), which are relaxed in the case of other consonants.

Further Reading

Take it from me: you're mad if you try to learn this language! But if you want to join me in the madhouse, Thomas Lambdin's Introduction to Sahidic Coptic is the standard textbook.