Neutral barrels are those that have been in service long enough to have used up most of their oak extraction, thus becoming “neutral” in the oak flavors imparted to the wine. Though slower to imbue the wine with significant oak flavors, these barrels are good, clean wine aging and storing vessels that will impart oak flavors and micro-oxygenation to the wine at a slower rate than newer barrels. Think of the barrel as an oak ingredient in the wine that is consumed more rapidly during the time the first fill is in the barrel and is consumed at a much slower pace after 4-5 years use.
*Neutral Black barrels are those with significant dark staining on the outside that is a result of having been stored in humid underground wine caves. The barrels appearance is sometime less desirable than a “newer” looking barrel, but the inside of the barrel (where it matters) can be just as clean and sanitary as newer “looking” barrels of the same or newer vintage.