How do you convert the vacum to a blower?

Go to B&D website. You can get them there.

Bc 4000 Manual downloadthingsnow com

250 MPH Electric Blower Vac Mulcher ' BV5600 ' BLACK+ ...

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Black & DeckerThe High Performance 12 amp motor 3-in-1 blower, vacuum, and mulcher keeps ....blower I was using and I let him use it and he liked it alot said he'd never buy the ... So thanks again Black & Decker for making a safe usable product that will ...

Black and Decker BV3600 12 Amp Blower Vac - Amazon.com

www.amazon.com > ... > Leaf Blowers & Vacuums
Amazon.com, Inc. Rating: 4 - ‎138 reviews
Greenworks 24012 7 Amp Single Speed Electric 160 MPH Blower B0030BG1CC ...The Black & Decker BV3600 Blower Vac has a powerful 12 amp motor. .... really well, so that I had about 4 bags of debris to empty, and I have a big lawn.

Couldn't find one. Might this one serve?

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CDIQFjACahUKEwi3s7mfpIvGAhVRMowKHbmUAP0&url=http%3A%2F%2Fwww.ehow.com%2Fhow_7484314_thread-black-decker-st6000.html&ei=wWd7VffOFtHksAS5qYLoDw&usg=AFQjCNFL3IyFWdUOOtb62OxVuFd6dzbi7Q&sig2=mPBwzo9n_QxBGoVh_P5efg&bvm=bv.95515949,d.cWc

gas or electric ???
gas, check for a plugged exhaust and learn how to mix your fuel correctly
electric, throw that thing away

Try Platt Electrical supply - 800-257-5288 (Platt.com) BLACK & DECKER #89754-01 REPLACEMENT BRUSH. It seems the entire motor head was manufactured by DeWalt and is now obsolete. I don't know if these brushes will fit, but someone there may help.
Good Luck!

When the machine is converted to vacuum mode, the collection bag will go on the machine where the blower tube (long part) is connected for blowing mode.

Open the bottom vent where the air comes in...there should be a locking clip there that when depressed with a screw driver will allow it to open...Install the vacuum tube in the opening made by opening the vented bottom...then remove the plastic blower tube and install a collection bag on the outlet where the tube was.
Please vote on this solution.

I have not heard of one for that unit. Plus I would not use it for vacuuming any way. All you need is for an acorn or piece of wood to hit the impeller.
It would break a couple of fins off the impeller. This would make the blower vibrate like crazy and you will not want to hold on to it. Plus the cost of repairs would exceed the cost of a new one.
Good Day.