It is permissible to perform ablution while having a bandage. If it does not harm the wound, water may be passed over it. However, if doing so would cause harm or delay the healing process, it is sufficient to wipe over the bandage (masah).
If the condition suggests that bleeding is still ongoing beneath the bandage, then ablution should be performed in the manner applicable to a person with a continuous excuse. However, if what appears is merely old blood softening due to contact with water, this does not invalidate the ablution, as it is not considered fresh bleeding.