This term refers to the conversion of Fender Silverface (SF) amps (which were built from around 1968 to around 1980) to the more desirable Blackface (BF) configuration (circa 1964 to 1967). Most SF Fenders can be converted to BF spec. for much lower cost than purchasing a vintage BF model. There are some exceptions, mainly the "ultra-linear" amps with 70 watt and 135 watt versions of some models.
Blackface amps were made with a black faceplate and featured the pre CBS circuit design used by Fender before the advent of the silver faceplates. It featured a better bias circuit that allowed for more precise bias settings for your output tubes. Blackface models also had some different value components in various places throughout the amp which resulted in what most players consider to be a better tone. The number of changes varies according to amp model and year.
When we blackface your amp, we change these components, re-bias the amp, and on some amps we make changes to the wiring. Selected wires are re-routed and/or shielded to to stabilize the amp so that it will not become unstable after changing the components.
These wiring changes are often necessary because some SF amps become somewhat unstable after converting to blackface spec. This is because CBS made changes to the SF wiring to lower manufacturing costs. These changes sometimes caused instability in the amp which required them to make other circuit changes to offset the new wiring problems. These circuit changes fixed the instability but adversely effected the tone of the amp. This is one of the reasons why blackfacing can improve the sound of your amp.
Many customers ask what tone changes they can expect after getting the blackface conversion. That really depends on the amp being converted. The amps that can benefit the most from this mod are the late '68 through late '69 versions of certain models. These are the amps that gave silverface Fenders a bad name when there were introduced. Blackfacing these amps results in a more classic Fender tone with better dynamics, more controllable overdrive, extended frequency response, slightly less flabby bass, and clean tones that have a more open and sparkly sound. In addition, the bias circuit will be converted to a standard blackface type that allows for precise adjustment without pulling the amp chassis. This saves time and money on future output tube replacements. For most silverface amps not built in the above time period, you still get the bias adjustment mod but the other changes will be a bit more subtle. Basically, the tone will get a bit more harmonic, very slightly more distorted in a good way. However, some players may prefer the tone of an unmodified '70s silverface amp which generally have a really great tone if working properly.
Pricing for this service varies but is generally about $75 - $85 for most late 60s and early 70s Silverface Fender amps. Master Volume and high power models such as Twins and Showmans can be a little higher depending on how you want to deal with master volume controls and pull/boost controls. Ultra-linear amps can be considerably more and still won't really sound like blackface amps because of the different transformers in these amps.