They are both correct English. As far as I am aware echo questions are not a formal construct (edit: by which I mean that they are not syntactically distinct) and both forms are perfectly acceptable on their own. They can be used sarcastically as well to feign surprise. To me, the key is that while the '?' distinguishes them as questions in written work, when spoken one can only tell the later is a question by an inflection in the intonation that is unwritten. While this kind of question is common in English, the use of intonation to convey meaning is not generally common in English, though it is in some other languages, e.g. Mandarin.
What I mean is, it makes perfect sense to walk into a room and say "John left already?" This is still the case even if you're not surprised that you don't see John and no one said anything before your question.
Perhaps another aspect is context in regards to how likely it is that you are to ask a question that way. That is, if you are going out with friends by train, even though no one has mentioned tickets you might say, "we have tickets?" or "you have the tickets?" while approaching the station. Adding the interrogative will often make your question more clear though, especially in cases with less context.
Edit: My examples here are so called declarative questions, which need not be (though are commonly) echoic.
"The guiding hypothesis in explaining the restriction is that
questions must be uninformative with respect to the Addressee
- a requirement that declaratives can only meet in certain contexts. The analysis predicts , correctly, that in addition
to their familiar "echoing" function, rising declaratives may
be used to question presuppositions and inferences taken to
follow from the Addressee's public position, whether or not
such inference finds its basis in a preceding utterance."
(17) [A&B are looking at a co-worker's battered and dented car]
A: His driving has gotten a lot better.
a. Has it? I don't see much evidence of that.
b. It has? I don't see much evidence of that.
c. It has. #1 don't see much evidence of that.
This skeptical reading of rising declaratives is well known, and is often
assumed to be connected to their "echoing" function. But it would be a
mistake to assume that rising declaratives are inherently skeptical
(or inherently echoing, for that matter). Rising declaratives, like
interrogatives, also allow for readings in which the Speaker is
understood as routinely accepting the proposition expressed, as
illustrated in (18), where the falling declarative is acceptable as
A: That copier is broken.
a. Is it? Thanks, I'll use a different one.
b. It is? Thanks, I'll use a different one.
c. (Oh), it is. Thanks, I'll use a different one.
University of California, Los Angeles
Available here: http://journals.linguisticsociety.org/proceedings/index.php/SALT/article/viewFile/2860/2600
Realized that I did not directly answer the question. In short, "We have a blog?" is correct and does have a different meaning than "Do we have a blog?" In this instance it probably does indicate surprise as some other answers have mentioned. My reason for writing this answer was to point out that this is not always the meaning conveyed by this syntactic structure.