The baseball draft eligibility rules can be a bit quirky, and have several differentiators from basketball/football.
The first big difference is that you do not declare for the MLB draft. If you are eligible, you may be selected. This does not cost you eligibility or mean that you have to sign. For the most part, you are draft eligible after your senior high school season (if you graduate early, under a certain age, etc., there are some restrictions - Bryce Harper was an example of this). If you go the college route, there are two paths with their own draft eligibility rules - JUCO and 4-year. If you attend a JUCO, you are draft eligible after every/any season. If you attend a 4-year school, you are draft eligible 3 years after your high school graduation (junior/redshirt soph year) and beyond. If you turn 21 within so many days (I think it's 90 maybe?) of the draft, you are draft eligible as a sophomore (Marin & Kuhns this year, Curley last year).
Due to leverage with signing bonuses, it is typically in the best interest of the player to be drafted and sign early (hence draft eligible sophs going when selected). Guys out of eligibility - such as Chapman, Blanco - will not have much leverage since they can't just decline to sign and go back to play another year in college. MLB/MLBPA collective bargaining agreement expires this coming offseason and many expect a work stoppage next season. That possibility will throw a wrinkle in the decisions of drafted kids with remaining eligibility (will there be MLB baseball next year, will draft bonuses go up/down based on new agreement, will there be fewer minor league teams/levels, will number of rounds in draft change, etc.).
