I want to erase parts of a squre to make look like a 90 degrees V, but when I use the eraser tool it turns everything to black.
How can I fix that?
Photoshop is using your Background color, which is currently Black. Use the keyboard shortcut of “D” to reset to the default colors.
After resetting the colours, the eraser turns it to black instead of blue...how do I make it transparent?
If you are on a Background layer, the Background layer cannot be transparent. It will erase to your Background Swatch. Type "D" to reset the swatches to black and white.
Convert the Background layer to a regular pixel layer by clicking the lock icon in the Layers panel. Now you can erase to transparency.
A far better idea, though, is to add a layer mask (Layers panel) and use the Brush tool to paint with black or white on the mask (not on the layer) to hide or show the pixels. This is non-destructive, as you can repaint your mistakes with the opposite color.
If the layer is locked it will erase to the background colour
Just to add to Jane's correct answer.
On a normal layer, the eraser tool will delete pixels i.e. turn them transparent. However, on a background layer (which is a specific type of layer) pixels cannot be transparent, so instead the eraser paints with the current background colour.