The straightforward answer is probably. As I'm assuming you know since you asked this question, your blood is produced in the bone marrow. However, most of it pretty immediately goes into your bloodstream, though there will still be some residual proteins which have been made but haven't been shipped yet. The main cell type that actually stay put in your bone marrow for any length of time are the immune cells responsible for keeping a record of invaders that your body has previously faced (B cells). There are also a lot of things in your blood at any given time that are not produced in your bone marrow (hormones, oxygen, medications, alcohol, bloodborne pathogens, etc.) so depending on what is the essential part of your blood a vampire actually needs for nutrition, it may or may not be found in the bone marrow, and may or may not be in any relevant quantity. Would probably still taste good, though, even if it wasn't nutritionally beneficial.
And according to Procopio Et al (2018), the timeline is very dependent on environmental conditions (the drier the bones, the better the preservation, this is why we have so much evidence of what the Egyptians were up to 6000 years ago, the place is just really dry so things don't decompose much). They note that the rate of decay is generally most noticeable in the 1-4 month range, with levels reaching their minimum at about 9 months, and their experiment was conducted in England so it was pretty rainy. The methods they used were measuring the proteosome, so really interesting to me I'm glad you sent me in this direction.