1 to greet or acknowledge, but only slightly
2 to express sentiment or mood, but only slightly
Not to be confused with blah the term bleh is generally utilized as a conversation deterrent. Oftentimes people find themselves at the mercy of a conversation they do not wish to participate in. This predicament is typically initiated by a person greeting another or by a person answering a question regarding their general state of health/well being with what might be called a conversation initiator. For example, if person A were to ask person B how he/she is doing, the answer might take the form of an unwanted 30-minute conversation. It is in situations like this, where there is pressure to acquiesce a measure of social etiquette with a response but insufficient time/desire to address the situation with the full answer as would normally be offered that we have use for this term. In definition 1 then the implied sentiment, if transcribed, would be, "I acknowledge that you still exist...that's great...good work...I'd love to take the time to greet you properly but I simply cannot at the moment." (Note: it's possible that, in fact, you would not love to take the time to greet that person properly; in any instance)
Regarding definition 2, intonation is an important variable. Again the same theme of deterring a conversation is at heart, but in the interests of satisfying social etiquette the term may be used with varying intonation to reveal some extra detail as to the person's sentiment/mood. A short descending tone might, for example, reveal anger (another reason why you should not try to have a conversation with that person), while a stretched monotone might be used to express boredom or discontent. With experience this term can come to have a variety of specific meanings when accompanied by a mastery of its applicable intonation.